Fix operator spacing.

cabextract: WARNING lines aren't fatal

Stdenv: Add ENV.libxml2 and ENV.x11 for Linux

Add vendored sha256

The warnings this prints makes it not especially helpful, so
just assume the magic number test and job failure are enough.

Fixes #2689.

But still prefer shasum/sha256sum where present.

Digest::SHA2 wasn't exposed under that name in older versions of Ruby.

Update jenkins.brew.sh links.

These previously, incorrectly pointed to bot.brew.sh.

Fixes #2703.

audit: Port audit_text method to rubocop and add tests

Add ENV.libxml2 primarily for the use of test do blocks.
Add a dummy ENV.x11 function.

See Linuxbrew/brew#356 and Linuxbrew/brew#382

Fix Linux build

Remove `testbottest` formula (this should have lived in the `brew
test-bot` repository from the outset) and make various other changes to
fix the Linux build here.

tests: install specific Bundler version.

check_non_libraries: fix false positive subdirectory reports

Otherwise `brew tests` fails with the latest. See the failing
Homebrew/homebrew-test-bot `master` build as an example.

Use parallel RuboCop

This requires updating to Rubocop 0.49.0 which will require some fixes
to rules, in Homebrew/brew and Homebrew/homebrew-core but opening this
for now so I remember.

Convert `FormulaLock` to more generic `LockFile`.

Use `Formatter::pluralize` where possible.

update-test: tweak more edge cases.

Don't try to `fetch --depth=1` unless we've confirmed the repository is
already shallow. Otherwise this can discard history from a full clone.

Additionally, if the repository wasn't shallow but there's no tags found
assume that Git is too old and try a naive sort if on Linux where `sort`
is able to sort versions.

update-test: improve no tags found messaging.

Currently you just get `Could not find start commit!` which is not as
explicit as it could be.

Improve some `brew install` messaging.

Match the "formula name in description" on word boundaries

Use `XDG_CACHE_HOME` for RuboCop.

- The regexp for the "check if formula name is used in formula's
  description" cop matches every instance of the formula name if it
  exists, whether it's in a word or not.
- For example, the formula `mon` has the description "Monitor
  hosts/services/whatever and alert about problems". This makes
  `brew audit --strict` complain because it matches "Monitor",
  which isn't the formula name! The formula `pass` has the description
  "Password manager".  Again, the strict audit matches "Password",
  which isn't an issue.
- Instead, this change matches on a word boundary, so it will match
  `mon:`, or `mon `, but not "Monitor", or, for example, "harmony".
- I've changed the tests to account for this change.

diagnostic: move some more macOS doctor checks.