in fm_bot, use -foo to remove a source

We're not using -foo today to do anything but print a "sorry" error.
Might as well make it something productive.

In this case, it happens that Codec_PngRoundTrip is flaky on my laptop;
I'm guessing the blame belongs with CoreGraphics.  Anyway, this lets me
skip it but run all other tests with a line like

    tests -Codec_PngRoundTrip b=cpu

Change-Id: I479a71575eadfc1f870a3c48c130b2b57797ac54
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/317316
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed