[SPARK-18422][CORE] Fix wholeTextFiles test to pass on Windows in JavaAPISuite

## What changes were proposed in this pull request?

This PR fixes the test `wholeTextFiles` in `JavaAPISuite.java`. This is failed due to the different path format on Windows.

For example, the path in `container` was

```
C:\projects\spark\target\tmp\1478967560189-0/part-00000
```

whereas `new URI(res._1()).getPath()` was as below:

```
/C:/projects/spark/target/tmp/1478967560189-0/part-00000
```

## How was this patch tested?

Tests in `JavaAPISuite.java`.

Tested via AppVeyor.

**Before**
Build: https://ci.appveyor.com/project/spark-test/spark/build/63-JavaAPISuite-1
Diff: https://github.com/apache/spark/compare/master...spark-test:JavaAPISuite-1

```
[info] Test org.apache.spark.JavaAPISuite.wholeTextFiles started
[error] Test org.apache.spark.JavaAPISuite.wholeTextFiles failed: java.lang.AssertionError: expected:<spark is easy to use.
[error] > but was:<null>, took 0.578 sec
[error]     at org.apache.spark.JavaAPISuite.wholeTextFiles(JavaAPISuite.java:1089)
...
```

**After**
Build started: [CORE] `org.apache.spark.JavaAPISuite` [![PR-15866](https://ci.appveyor.com/api/projects/status/github/spark-test/spark?branch=198DDA52-F201-4D2B-BE2F-244E0C1725B2&svg=true)](https://ci.appveyor.com/project/spark-test/spark/branch/198DDA52-F201-4D2B-BE2F-244E0C1725B2)
Diff: https://github.com/apache/spark/compare/master...spark-test:198DDA52-F201-4D2B-BE2F-244E0C1725B2

```
[info] Test org.apache.spark.JavaAPISuite.wholeTextFiles started
...
```

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #15866 from HyukjinKwon/SPARK-18422.