glibc-tests: correctly pull in the actual tests when installing -ptest package

The tests are packaged into the main glibc-tests package which is fine,
but then glibc-tests-ptest package needs to depend on that.
Which is what this commit addresses.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit d37c2d42)
Signed-off-by: Steve Sakoman <steve@sakoman.com>