Fix bug in vanilla-comp-test-actions

Improved diagnostics when a should_compile test fails.

More diagnostic wibbles.

Further improvements to printing of diagnostic information.

normalise_errmsg()s in a few more places.

Further adventures to do with approximate matching of compiler error
messages.  Use the new pipe facility in the test driver to normalise
error messages before comparison, by piping them through new program
normalise_errmsg.  This avoids various kinds of tiresome comparison
failures to do with capitalisation, whitespace changes, and path
names.

Add support for compile-only (should_run) style tests.

Track recent hackery.

Commit hackery so far.

Changes to support running "vanilla" tests -- those with a single source
file, reference .stdin, .stdout and maybe .stderr files.  These constitute
the vast majority of GHC tests.