tf-a-tftf-main: fix the REPO_UNDER_TESTS name

Renamed REPO_UNDER_TESTS to 'tf-a-tests' to match the repository
name in the clone.sh script.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I632fbcbd96d302364bbfa8da4dbeef01fa29546c
1 file changed