Cleanup: tests: use find's `-name` option instead of grep

Cleanup: tests: use find's `-name` option instead of grep

This prevents grep of finding matches in the leading directories listed
by find.

It caused problem when I used the "metadata" string in one of the
directory names:
  /tmp/tmp.test_regen_metadata_ust_trace_path.6FbwPF/

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I7165a3b4e4a16c793fe51c40484978be3a99dbfc