tools/mvtest: ensure testcase is executable (755)

tools/mvtest: ensure testcase is executable (755)

Some test cases lack executable permission ('x'). Before running each
test case, `./check` checks and grants them 'x' permission. However,
this always leads to a dirty git repo. And the absence of 'x' permission
in test cases is often overlooked during reviews.

Since maintainers use mvtest to assign new case, add this change for
convenience of maintainers.

Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>