enable SwiftifyImports/CxxSpan tests (NFC)

[hnrklssn]

These tests require the 'std_span' feature, but that feature was previously only added in the test/Interop test suite. As a result these tests would never run.

[hnrklssn]

@swift-ci please smoke test

[j-hui]

It seems like you are missing this definition:

def get_target_os():
    import re
    (run_cpu, run_vendor, run_os, run_version) = re.match('([^-]+)-([^-]+)-([^0-9]+)(.*)', config.variant_triple).groups()
    return run_os

[hnrklssn]

@swift-ci please smoke test

[hnrklssn]

@swift-ci please smoke test