Add check for path separator to TempDir

[leonrosenshein]

Since GO 1.17, ioutil.TempDir has called MkdirTemp for it's implementation. Since at least October of 2020, MkdirTemp has checked the input prefix for os.PathSeparator. This PR adds that check and returns an error in that case.

The PR also includes some basic tests for the TempDir function.

[CLAassistant]

All committers have signed the CLA.