versioner: merge stdout and stderr in the test runner.

Some of the error messages emitted by versioner (the ones where it was
invoked incorrectly) go to stderr, which meant that the test runner
ignored them. Merge stdout and stderr, and switch from testing for
exact equality to endswith, because of the compilation errors test.

Change-Id: I0e2c25bcc9dea4c12ea82a6a05b29e561a61a902