Fix MB and running browser_tests under mash
Unfortunately, the way to run a test on the bots can vary between
swarming and running directly; to run under swarming, you need
to ensure that the files needed to run the test and the command
line is embedded in the BUILD files (and gn_isolate_map) themselves,
rather than being able to specify them in the //testing/buildbot files.
This patch changes the way we were attempting to run tests
for mash on the chromium.mojo linux bot, so that the tests
are run under swarming.
In addition, this patch fixes a bunch of bugs uncovered when
trying to reproduce issues locally, and improves the help