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
(bazelbuild/bazel@2780393).

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

Author: fmeum

tests/base_rules/py_executable_base_tests.bzl

@@ -356,6 +356,7 @@ def _test_main_module_bootstrap_system_python(name, config):
         target = name + "_subject",
         config_settings = {
             BOOTSTRAP_IMPL: "system_python",
+            "//command_line_option:extra_execution_platforms": ["@bazel_tools//tools:host_platform", LINUX_X86_64],
             "//command_line_option:platforms": [LINUX_X86_64],
         },
         expect_failure = True,
@@ -380,6 +381,7 @@ def _test_main_module_bootstrap_script(name, config):
         target = name + "_subject",
         config_settings = {
             BOOTSTRAP_IMPL: "script",
+            "//command_line_option:extra_execution_platforms": ["@bazel_tools//tools:host_platform", LINUX_X86_64],
             "//command_line_option:platforms": [LINUX_X86_64],
         },
     )