I noticed this while preparing for a new release of sed.

I don't like copying the 4-term #if into so many files, but it already
appears in ten others. Note that this covers only the gnulib tests
used by sed. This probably deserves a full audit and/or auto-run tests
with gl_DISABLE_THREADS, but I am stopping with the proposed patch
below.
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tests: skip thread-using tests when threading is disabled

sed's configure.ac specifies gl_DISABLE_THREADS, and that caused three
thread-using gnulib tests to fail. Add an #if-guarded exit (77) to each
of those, so they are skipped in this case.
* tests/test-nl_langinfo-mt.c (main): Exit 77 when threading is disabled.
* tests/test-setlocale_null-mt-all.c (main): Likewise.
* tests/test-setlocale_null-mt-one.c (main): Likewise.