Update llvm nightly tests to run spectests.

It turns out nightly does not build the spec tests unless -examples is set. So,
instead of setting the start_test dirs to "release/examples extern/ferguson",
just set extern/ferguson and call nightly with -examples flag.
Merge pull request #802 from thomasvandoren/run-spec-tests-for-llvm

Update llvm nightly tests to run spectests.