LU-9474 tests: fix quoting in stack_trap

LU-9474 tests: fix quoting in stack_trap

stack_trap() mishandled single quotes. This patch is not the cleanest
of fixes, but it works.

(sanity-hsm is the only test suite that uses the function, for now)

This patch is back-ported from the following one:
Lustre-commit: 99420a1830b89a8aba6350b095065d65107f7c0f
Lustre-change: https://review.whamcloud.com/30490

Test-Parameters: trivial testlist=sanity-hsm

Signed-off-by: Quentin Bouget <quentin.bouget@cea.fr>
Change-Id: Ia43219e57079abdbfc75485105d572bbfa85caba
Reviewed-on: https://review.whamcloud.com/34158
Tested-by: Jenkins
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>