Fix flaky tests

[dserfe]

This PR is to fix flaky tests.

Problem

If test jnr.posix.EnvTest#testSetenvOverwrite runs before any of the following 6 tests, it will pollute the shared status, thus these 6 tests will fail:

jnr.posix.GroupTest#getgroups,
jnr.posix.NlLanginfoTest#testNlLanginfo,
jnr.posix.SpawnTest#closeInput,
jnr.posix.SpawnTest#inputPipe,
jnr.posix.SpawnTest#outputPipe,
jnr.posix.SpawnTest#validPid

For example, we get the following test failures:

[ERROR] Failures: 
[ERROR]   SpawnTest.closeInput:182
[ERROR]   SpawnTest.inputPipe:107
[ERROR]   SpawnTest.outputPipe:73
[ERROR]   SpawnTest.validPid:44
[ERROR] Errors: 
[ERROR]   GroupTest.getgroups:92->exec:122 » IO Cannot run program "id": error=2, No such file or directory
[ERROR]   NlLanginfoTest.testNlLanginfo:28 » IO Cannot run program "locale": error=2, No such file or directory

Root cause and fix:

Line 38 of jnr.posix.EnvTest#testSetenvOverwrite changes the property related to "PATH" to "new value", but at the end of the test, it didn't reset it back. Therefore, the fix is to change it back at the end of test, which can clean the pollution to other tests.

[enebo]

@dserfe yeah good catch. Thanks for your contribution!