glsl/tests/glcpp: reinstate "error out if no tests found"

With the recent rework of converting the shell script to a python one
the check for actual tests was dropped.

Bring that back, since it was explicitly added considering we had a ~2
year period, during which the tests were not run.

v2: use raise Exception() over  print() & return false (Dylan)

Fixes: db8cd8e3 ("glcpp/tests: Convert shell scripts to a python
script")
Cc: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit d589eddc)