Reimplement in terms of the support for this feature in the makefiles with

the "check-one" target. This was necessitated by the change in the test
harness to fully use Tcl's "exec" function and avoid shell scripts
altogether. Consequently, to be accurate, TestRunner.sh needs to invoke the
same environment as dejagnu would, but for just a single test. That's what
the make target "check-one" does so we just invoke it here.

llvm-svn: 36051