tests: make some analysis tests work for when test's exec platform is required (#2869)

An upcoming change in Bazel makes the test toolchain required, which
means a compatible
exec platform amongst toolchains must be found
(https://github.com/bazelbuild/bazel/commit/2780393d35ad0607cf5e344ae082b00a5569a964).

Some analysis tests of `py_test` force the target platform to a specific
platform, but before this change didn't register a compatible exec
platform. This can be fixed by registering the target platform as an
exec platform. Since Python targets currently depend on a C++ toolchain
through Bazel's `launcher` and `launcher_maker` and the default
toolchain can't cross-compile to Linux, the host platform still needs to
be kept at highest priority to ensure that cross-compilation isn't
needed on macOS.

Work towards #2850