[lit] Add a test for the builtin config map.

Config map is not exposed through the command line, so testing this
is somewhat tricky.  But basically we need a test that if a custom
driver builds a config map and passes it to main, it gets respected.

A config map allows config files in the source tree to be mapped
to alternate config files in the build tree.  This particular test
works by having two config files in separate directories, and
setting up a config map to have that redirects A/lit.site.cfg
to B/altconfig.  Then, we print a message in A/lit.site.cfg
and B/altconfig and check that we do see the output from B
but don't see the output from A.  Additionally we test that
the test suite specified by A's config map is properly discovered.

Differential Revision: https://reviews.llvm.org/D38105

llvm-svn: 313887