[SPARK-8603][SPARKR] Use shell() instead of system2() for SparkR on Windows

## What changes were proposed in this pull request?

This PR corrects SparkR to use `shell()` instead of `system2()` on Windows.

Using `system2(...)` on Windows does not process windows file separator `\`. `shell(tralsate = TRUE, ...)` can treat this problem. So, this was changed to be chosen according to OS.

Existing tests were failed on Windows due to this problem. For example, those were failed.

  ```
8. Failure: sparkJars tag in SparkContext (test_includeJAR.R#34)
9. Failure: sparkJars tag in SparkContext (test_includeJAR.R#36)
```

The cases above were due to using of `system2`.

In addition, this PR also fixes some tests failed on Windows.

  ```
5. Failure: sparkJars sparkPackages as comma-separated strings (test_context.R#128)
6. Failure: sparkJars sparkPackages as comma-separated strings (test_context.R#131)
7. Failure: sparkJars sparkPackages as comma-separated strings (test_context.R#134)
```

  The cases above were due to a weird behaviour of `normalizePath()`. On Linux, if the path does not exist, it just prints out the input but it prints out including the current path on Windows.

  ```r
# On Linus
path <- normalizePath("aa")
print(path)
[1] "aa"

# On Windows
path <- normalizePath("aa")
print(path)
[1] "C:\\Users\\aa"
```

## How was this patch tested?

Jenkins tests and manually tested in a Window machine as below:

Here is the [stdout](https://gist.github.com/HyukjinKwon/4bf35184f3a30f3bce987a58ec2bbbab) of testing.

Closes #7025

Author: hyukjinkwon <gurwls223@gmail.com>
Author: Hyukjin Kwon <gurwls223@gmail.com>
Author: Prakash PC <prakash.chinnu@gmail.com>

Closes #13165 from HyukjinKwon/pr/7025.

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