test-container: Install with $(all-subdirs) [BZ #24794]

Whenever a sub-make is created, it inherits the variable subdirs from its
parent.  This is also true when make check is called with a restricted
list of subdirs.  In this scenario, make install is executed "partially"
and testroot.pristine ends up with an incomplete installation.

	[BZ #24794]
	* Makefile (testroot.pristine/install.stamp): Pass
	subdirs='$(all-subdirs)' to make install.

Reviewed-by: DJ Delorie <dj@redhat.com>