This basically rewrites every integration test. We reduce the amount of
magic involved here in terms of which arguments are being passed to
ripgrep processes. To make up for the boiler plate saved by the magic,
we make the Dir (formerly WorkDir) type a bit nicer to use, along with a
new TestCommand that wraps a std::process::Command. In exchange, we get
tests that are easier to read and write.
We also run every test with the `--pcre2` flag to make sure that works,
when PCRE2 is available.
