C#: Enable Semmle.Util.Tests.

[michaelnebel]

I think there are three issues causing the tests to fail when running them via bazel (one can also be provoked locally).

• When tests interact with the file system (eg. creating files) they should do so in a special folder: https://bazel.build/reference/test-encyclopedia#test-interaction-filesystem
• The tests are running in parallel on bazel (this is stated as a comment in our test runner). Since the tests were all interacting with the same file names (copying, creating and deleting) this could cause concurrency issues.
• When executing File.Delete(path) a directory not found exception can be thrown (this can happen for the "long" path in case the directory has not been created). This can also happen locally, if tests are not executed in the "right order" (som tests explicitly creates the folder containing the "long" path file).

In this PR we use the TEST_TMPDIR environment variable (when set) as the root folder for creating files and folders when running the tests. Furthermore, each test now uses a "unique" (random GUID) for long- and short filenames to avoid collisions when running in parallel and the constructor now initially creates the long path directory before we start running any tests (and some of the tests are adjusted accordingly).

[tamasvajk]

LGTM