[JENKINS-48407] Re-enable AtomicFileWriterTest#checkPermissionsRespectUmask() (#3275)

* [JENKINS-48407] Re-enable test

The previous test assumed permissions would always be the same,
when they actually depend on umask settings.

This change creates a file *not* using the temporary API, gets its
permissions then compares it to the ones obtained using
AtomicFileWriter.

Note: we now only check the given permissions, not the "non-given".

* Use assertThat(..., equalTo()) instead of a manual loop

* Remove unused imports

* Use TemporaryFolder instead of manual temporary dir creation