test: make FILEMODE a lazy prereq

test: make FILEMODE a lazy prereq

This way, test authors don't need to remember to source
lib-prereq-FILEMODE.sh before using the FILEMODE prereq to guard tests
that rely on the executable bit being honored when checking out files.

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>