xtest: add TempDir convenience method
xtest: add TempDir convenience method

This is more convenient than the existing `xtest.MustTempDir`, the name is chosen based on the test name.
While it is similar to the `testing.T.TempDir` method it allows to manually decide whether cleanup is desired or not.

GitOrigin-RevId: a1a98cb47a2994c5c922a86632d9a9acc7d652be

netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion