zmake: Add a helper function for test filenames
zmake: Add a helper function for test filenames

Create a new function to reduce the amount of duplicated code.

BUG=b:184298184
BRANCH=none
TEST=(cd zephyr/zmake/; python3 -m pytest tests/*.py -v -k test_filter)

Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Change-Id: I51124fa47eb298112bd9b2b06018d8f14b9968ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2880415
Reviewed-by: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec