[SPARK-18830][TESTS] Fix tests in PipedRDDSuite to pass on Windows

## What changes were proposed in this pull request?

This PR proposes to fix the tests failed on Windows as below:

```
[info] - pipe with empty partition *** FAILED *** (672 milliseconds)
[info]   Set(0, 4, 5) did not equal Set(0, 5, 6) (PipedRDDSuite.scala:145)
[info]   org.scalatest.exceptions.TestFailedException:
...
```

In this case, `wc -c` counts the characters on both Windows and Linux but the newlines characters on Windows are `\r\n` which are two. So, the counts ends up one more for each.

```
[info] - test pipe exports map_input_file *** FAILED *** (62 milliseconds)
[info]   java.lang.IllegalStateException: Subprocess exited with status 1. Command ran: printenv map_input_file
[info]   at org.apache.spark.rdd.PipedRDD$$anon$1.hasNext(PipedRDD.scala:178)
...
```

```
[info] - test pipe exports mapreduce_map_input_file *** FAILED *** (172 milliseconds)
[info]   java.lang.IllegalStateException: Subprocess exited with status 1. Command ran: printenv mapreduce_map_input_file
[info]   at org.apache.spark.rdd.PipedRDD$$anon$1.hasNext(PipedRDD.scala:178)
...
```

`printenv` command prints the environment variables; however, when environment variables are set to `ProcessBuilder` as lower-cased keys, `printenv` in Windows ignores and does not print this although it is actually set and accessible. (this was tested in [here](https://ci.appveyor.com/project/spark-test/spark/build/208-PipedRDDSuite) for upper-cases with this [diff](https://github.com/apache/spark/compare/master...spark-test:74d39da) and [here](https://ci.appveyor.com/project/spark-test/spark/build/203-PipedRDDSuite) for lower-cases with this [diff](https://github.com/apache/spark/compare/master...spark-test:fde5e37f28032c15a8d8693ba033a8a779a26317). It seems a bug in `printenv`.
(BTW, note that environment variables on Windows are case-insensitive).

This is (I believe) a thirdparty tool on Windows that resembles `printenv` on Linux (installed in AppVeyor environment or Windows Server 2012 R2). This command does not exist, at least, for Windows 7 and 10 (manually tested).

On Windows, we can use `cmd.exe /C set [varname]` officially for this purpose. We could fix the tests with this in order to test if the environment variable is set.

## How was this patch tested?

Manually tested via AppVeyor.

**Before**
https://ci.appveyor.com/project/spark-test/spark/build/194-PipedRDDSuite

**After**
https://ci.appveyor.com/project/spark-test/spark/build/226-PipedRDDSuite

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #16254 from HyukjinKwon/pipe-errors.