(maint) Use PuppetSpec::Files for tmpdir creation in tests.

Before this change, the PMT Checksummer spec was using the in-built
'tmpfile' library directly.  For conformance with the rest of the test
suite, it's been recommended that we should use PuppetSpec::Files for
that functionality (which this change implements).
(maint) Fix metadata validation for missing namespaces and module names.

Prior to this commit, validation would raise non-sensical validation
errors if you attempted to validate a module name that contained a
separator (either '-' or '/'), but less than one character on either
side.  This change adds validation for those cases.