add test to verify that in-tree tooltool.py copies match this one

[bhearsum]

We have 3 different copies of the tooltool.py client vendored into the tree:

• python/mozbuild/mozbuild/action/tooltool.py
• testing/mozharness/external_tools/tooltool.py
• testing/web-platform/tests/tools/third_party/tooltool/tooltool.py

The last time we tried to update the in-tree copies, we discovered that two of them had differences not found in the other, nor the copy in this repo.

We should add a test here to verify that all copies are the same. Maybe a test in tree as well.

[gbrownmozilla]

testing/mozharness/external_tools/tooltool.py has been eliminated - one less copy to worry about.