Made self.expect() errors a little more readable in the testsuite.

self.expect() had two problems:

- If there was a substrs argument, then it overwrote the variable containing
  the command to run with the last substr.  That meant nonsense command text in
  testsuite errors.

- The actual output is not printed, which makes fixing testsuite failures a bit
  annoying (you end up having to use the -tv arguments to dotest).

This fixes both of these issues.  We could do even better, pretty-printing the
criteria for "correct" output, but this at least makes dealing with errors a bit
better.

llvm-svn: 262950