[SPARK-21128][R] Remove both "spark-warehouse" and "metastore_db" before listing files in R tests

## What changes were proposed in this pull request?

This PR proposes to list the files in test _after_ removing both "spark-warehouse" and "metastore_db" so that the next run of R tests pass fine. This is sometimes a bit annoying.

## How was this patch tested?

Manually running multiple times R tests via `./R/run-tests.sh`.

**Before**

Second run:

```
SparkSQL functions: Spark package found in SPARK_HOME: .../spark
...............................................................................................................................................................
...............................................................................................................................................................
...............................................................................................................................................................
...............................................................................................................................................................
...............................................................................................................................................................
....................................................................................................1234.......................

Failed -------------------------------------------------------------------------
1. Failure: No extra files are created in SPARK_HOME by starting session and making calls (test_sparkSQL.R#3384)
length(list1) not equal to length(list2).
1/1 mismatches
[1] 25 - 23 == 2

2. Failure: No extra files are created in SPARK_HOME by starting session and making calls (test_sparkSQL.R#3384)
sort(list1, na.last = TRUE) not equal to sort(list2, na.last = TRUE).
10/25 mismatches
x[16]: "metastore_db"
y[16]: "pkg"

x[17]: "pkg"
y[17]: "R"

x[18]: "R"
y[18]: "README.md"

x[19]: "README.md"
y[19]: "run-tests.sh"

x[20]: "run-tests.sh"
y[20]: "SparkR_2.2.0.tar.gz"

x[21]: "metastore_db"
y[21]: "pkg"

x[22]: "pkg"
y[22]: "R"

x[23]: "R"
y[23]: "README.md"

x[24]: "README.md"
y[24]: "run-tests.sh"

x[25]: "run-tests.sh"
y[25]: "SparkR_2.2.0.tar.gz"

3. Failure: No extra files are created in SPARK_HOME by starting session and making calls (test_sparkSQL.R#3388)
length(list1) not equal to length(list2).
1/1 mismatches
[1] 25 - 23 == 2

4. Failure: No extra files are created in SPARK_HOME by starting session and making calls (test_sparkSQL.R#3388)
sort(list1, na.last = TRUE) not equal to sort(list2, na.last = TRUE).
10/25 mismatches
x[16]: "metastore_db"
y[16]: "pkg"

x[17]: "pkg"
y[17]: "R"

x[18]: "R"
y[18]: "README.md"

x[19]: "README.md"
y[19]: "run-tests.sh"

x[20]: "run-tests.sh"
y[20]: "SparkR_2.2.0.tar.gz"

x[21]: "metastore_db"
y[21]: "pkg"

x[22]: "pkg"
y[22]: "R"

x[23]: "R"
y[23]: "README.md"

x[24]: "README.md"
y[24]: "run-tests.sh"

x[25]: "run-tests.sh"
y[25]: "SparkR_2.2.0.tar.gz"

DONE ===========================================================================
```

**After**

Second run:

```
SparkSQL functions: Spark package found in SPARK_HOME: .../spark
...............................................................................................................................................................
...............................................................................................................................................................
...............................................................................................................................................................
...............................................................................................................................................................
...............................................................................................................................................................
...............................................................................................................................
```

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18335 from HyukjinKwon/SPARK-21128.

R/pkg/tests/run-all.R

@@ -30,10 +30,10 @@ if (.Platform$OS.type == "windows") {
 install.spark()
 
 sparkRDir <- file.path(Sys.getenv("SPARK_HOME"), "R")
-sparkRFilesBefore <- list.files(path = sparkRDir, all.files = TRUE)
 sparkRWhitelistSQLDirs <- c("spark-warehouse", "metastore_db")
 invisible(lapply(sparkRWhitelistSQLDirs,
                  function(x) { unlink(file.path(sparkRDir, x), recursive = TRUE, force = TRUE)}))
+sparkRFilesBefore <- list.files(path = sparkRDir, all.files = TRUE)
 
 sparkRTestMaster <- "local[1]"
 if (identical(Sys.getenv("NOT_CRAN"), "true")) {