[fuzzer][test] Disable big-file-copy.test for everything but macOS

This test is heavy on test resources and involves moving a large 2GB+ file
across an ssh connection when testing on remote devices. Exclude all tests
except for macOS testing (on host). Remote device testing for macOS may
eventually cause a problem on macOS as well w/o better handling of tests
that require more resource/time than we currently handle.
    Introduced by:
        https://reviews.llvm.org/D146189
    Subsequent exclusion of all but darwin by:
        https://reviews.llvm.org/D147094
    Subsequent exclusion of tvOS and watchOS by:
        https://reviews.llvm.org/D147502
    Subsequent exclusion of all but macOS...

rdar://107570309

Reviewed By: yln, thetruestblue

Differential Revision: https://reviews.llvm.org/D148727