[SPARK-18842][TESTS][LAUNCHER] De-duplicate paths in classpaths in commands for local-cluster mode to work around the path length limitation on Windows

## What changes were proposed in this pull request?

Currently, some tests are being failed and hanging on Windows due to this problem. For the reason in SPARK-18718, some tests using `local-cluster` mode were disabled on Windows due to the length limitation by paths given to classpaths.

The limitation seems roughly 32K (see the [blog in MS](https://blogs.msdn.microsoft.com/oldnewthing/20031210-00/?p=41553/) and [another reference](https://support.thoughtworks.com/hc/en-us/articles/213248526-Getting-around-maximum-command-line-length-is-32767-characters-on-Windows)) but in `local-cluster` mode, executors were being launched as processes with the command such as [here](https://gist.github.com/HyukjinKwon/5bc81061c250d4af5a180869b59d42ea) in (only) tests.

This length is roughly 40K due to the classpaths given to `java` command. However, it seems duplicates are almost half of them. So, if we deduplicate the paths, it seems reduced to roughly 20K with the command, [here](https://gist.github.com/HyukjinKwon/dad0c8db897e5e094684a2dc6a417790).

Maybe, we should consider as some more paths are added in the future but it seems better than disabling all the tests for now with minimised changes.

Therefore, this PR proposes to deduplicate the paths in classpaths in case of launching executors as processes in `local-cluster` mode.

## How was this patch tested?

Existing tests in `ShuffleSuite` and `BroadcastJoinSuite` manually via AppVeyor

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #16266 from HyukjinKwon/disable-local-cluster-tests.