Add test driver for srb snapshot tests

[jez]

This change introduces a lot of code. I've taken a lot of care to make sure
that all the commands are robust, have good error messages, are easy to debug
when they fail, and generally do what you expect.

Please let me know if this is not the case.

Review by commit.

Commit summary

• Make all forks absolute (b058bab)

  We don't want to rely neither on things being on the path nor on gems
  being installed, because this makes it harder to test locally.

  Key observation: once any piece of our code is running (either srb or
  srb-rbi), we can always compute the path to any other component we
  need. This means we don't have to rely on rvm, rbenv, the PATH variable,
  etc.

  For most things, this is just a matter of sprinkling a bunch of ../../
  The one tricky thing is that if we can't find the path to sorbet in
  sorbet-static, we fall back to trying to find the version of Sorbet
  that bazel has built in bazel-bin/main/sorbet.

• Add new test harness (1b3842b)

  The main entrypoint is test/snapshot/driver.sh.
  There are two kinds of tests: total and partial snapshot tests.

  In a total snapshot test, the contents of the actual sorbet/ folder from
  srb init must match the entire contents of the expected sorbet/ folder,
  no more and no less.

  In a partial snapshot test, the only files that have to match are the ones
  mentioned in the expected/ folder. Extra files are ok. This is nice for
  being able to, say, omit the sorbet-typed folder, or only check the RBI for
  a specific gem.

  In both tests, we're currently ignoring hidden-definitions. These were
  already being ignored by our existing tests. We'll need to add these in a
  follow up.

• Convert test/srb-rbi/empty to a test/snapshot/total (55ee234)

  The new test driver is a superset of the functionality of the existing
  test/srb-rbi/empty test, so I've removed it and re-made it using the new
  framework.

• Add a partial test (c546e38)

  Just to verify that partial tests also work.

• Use test/snapshot/driver.sh in update script (eceba6b)

  To re-record the snapshots, we can use test/snapshot/driver.sh --update, or
  run the normal tools/scripts/update_exp_files.sh.

  It's possible that this becomes really slow in the future, and if so I think
  we should consider removing test/snapshot/driver.sh --update from the
  general-purpose update script.

• Test in CI (6634b56)

  We're currently only testing the un-packaged version of the gem.

  In a future change, I'll add functionality to the driver to either test the
  srb executable that's on your PATH, or test the local srb executable
  located at $PWD/bin/srb.

• Don't serialize bundler.rbi (476ae23)

  Also don't create sorbet/gems/ folder if would be empty.

  We can't have a Gemfile.lock ourselves, because we're a library, not a
  leaf package, which means we can't pin the version of bundler that we need
  to require to load a project's Gemfile. In light of this, we're going to
  have to rely on sorbet-typed to have RBI files for anyone in the community
  who wants to use bundler as a gem.

[jez]

I split out the CI portion into a separate job. ptal

[ptarjan]

are you going to fix this? If not, I'd not bother with the TODO

[jez]

I plan to fix this in literally the next PR I write. I want to keep the TODO there because it helps me remember which things are in progress.