test(toolchain): use flag_values in test platform defs (#2106)

This PR uses the `flag_values` from the platform definitions
for the toolchains so that in the future we can distinguish
between `musl` and `glibc` toolchains in our tests.

For now the change is no-op.

As part of this change we are also registering the coverage
tools so that we can run `bazel coverage` with no errors.

See comment on #2095.

tests/toolchains/defs.bzl

diff --git a/tests/toolchains/defs.bzl b/tests/toolchains/defs.bzl
index 076e6b4..fbb7082 100644
--- a/tests/toolchains/defs.bzl
+++ b/tests/toolchains/defs.bzl
@@ -26,6 +26,7 @@
     for platform_key, platform_info in PLATFORMS.items():
         native.config_setting(
             name = "_is_{}".format(platform_key),
+            flag_values = platform_info.flag_values,
             constraint_values = platform_info.compatible_with,
         )