Build: Add version.txt to track tf-m-tests version

Although auto-downloading tf-m-tests repo by CMake has been removed for
split build, it is still worth to track the recommended tf-m-tests
version in TF-M.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: Ic9873da01d01bfde6a5a1bf71b2758a109594e46
1 file changed