tests: make toolchain tests run in the same build instead of bazel-in-bazel integration tests

[rickeylev]

This changes the //tests/toolchains tests to run in the same Bazel build instance instead of
being bazel-in-bazel integration tests. This is done by using a transition to set
the python version to match the desired toolchain.

This makes running the tests much cheaper -- there were 37 sub-bazel processes being
started (one for each python version), which was very expensive. Debugging is also much easier because they aren't part of a bazel-in-bazel invocation.

[rickeylev]

Ok, CI Should be happy now.

@aignas Do we happen to have existing code to map the TOOL_VERSIONS/PLATFORMS platform info stuff to target_compatible_with values?

Over in tests/toolchains/defs.bzl, I'm generating a target for each version. However, not all versions support all platforms, e.g., 3.8.10 doesn't support windows. So I need to generate e.g.

py_reconfig_test(
  name = "python_3.8.10_test",
  target_compatible_with = select({
    <is x86_64-apple-darwin>: [],
    <is x86_64-unknown-linux-gnu>: [],
    ...etc, one entry for each TOOL_VERSIONS sha256 key...
    "//condiitons:default": ":incompatible",
  }),
  ...
)

That config setting generation looks awfully familiar, as do the platform 2-tuple strings (e.g. "aarch64-apple-darwin", "x86_64-unknown-linux-gnu").

[groodt]

LGTM

[aignas]

LGTM, thanks for simplifying!

[aignas]

Something like this could be super useful in registering minor_major versions by default.

[rickeylev]

Yeah, I had a similar thought. I'll post in slack.

[aignas]

We probably need to add:

Suggested change

	_create_toolchain_attrs_struct(python_version = v)
	_create_toolchain_attrs_struct(tag=struct(python_version=v, configure_coverage_tool = True, is_default=False))

To fix #2105. I'll include it in #2106.