As of Bazel 6.0.0, all Bazel-provided toolchains report consistent
compiler names that can be matched on with `select`. Having a central
place for `config_setting`s that can be used for this purpose makes it
so that rulesets don't have to define them themselves and moving the
compiler flag out of `@bazel_tools` becomes easier as it is no longer
referenced directly.
Also includes minor fixes to `//tests/...` so that it can be enabled in
CI.
