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

Commit 35e038c1 started to use an
incomplete list of subdirs based on $(all-subdirs) causing
testroot.pristine to miss files from nss.

Tested if the list of files in testroot.pristine remains the same.

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