[lldb] Fix and enable Windows minidump tests

SBFileSpec.fullpath always uses the forward slash to join the directory with the
base name. This causes mismatches when comparing Windows paths with backslashes
in two of the minidump tests. To get around that we just compare the directory
names separately from the filenames.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D81465