Enable fine-grained and universal exclusions for CoreFX tests

Enable fine-grained and universal exclusions for CoreFX tests

One problem we've had in coreclr Jenkins when running corefx
tests (in the coreclr outerloop runs -- the ones using the
run-corefx-tests.py harness) is the inability to exclude
failing tests for x86 or x64, and the inability to have
fine-grained exclusions. This has led to problems like
https://github.com/dotnet/coreclr/issues/22442 where
virtually all Windows x86 and x64 corefx runs fail due to
timeouts. And issues like https://github.com/dotnet/coreclr/issues/24159
where all corefx test legs fail, and we have no control
over exclusions to get them all passing again quickly.

Now that the corefx RunTests.cmd/sh wrapper scripts
parse named arguments, including a response file that
is passed on to xunit, and also that the corefx used
xunit has support for this response file as well as
fine-grained exclusions (per-method/per-class/per-namespace),
we can take advantage of it.

This change adds a single, global, corefx xunit exclusion
response file, that will be used for all platforms. Since this
run-corefx-tests.py mechanism is not expected to live much longer,
this seems sufficient.