Fix test that was never run.

Commit 83df1220 (r368334) added 'REQUIRES: linux' to this test, but
because triples are not respected by REQUIRES, that meant it was
invariably Unsupported.  The correct keyword would be 'system-linux'
(checking the host rather than the target).

Because the test was always skipped, commit 0cfd9e5b (r375439) did not
notice that the test modification was incorrect.

This patch corrects the REQUIRES clause and fixes the incorrect
previous patch.

Found after implementing https://reviews.llvm.org/D107162