test_cmp: diagnose incorrect arguments

Under normal circumstances, if a test author misspells a filename passed
to test_cmp(), the error is quickly discovered when the test fails
unexpectedly due to test_cmp() being unable to find the file. However,
if the test is expected to fail, as with test_expect_failure(), a
misspelled filename as argument to test_cmp() will go unnoticed since
the test will indeed fail, but for the wrong reason. Make it easier for
test authors to discover such problems early by sanity-checking the
arguments to test_cmp(). To avoid penalizing all clients of test_cmp()
in the general case, only check for missing files if the comparison
fails.

While at it, make test_cmp_bin() sanity-check its arguments, as well.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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