xfstests: make install support common/ and tests/ dirs

xfstests: make install support common/ and tests/ dirs

xfstests have some change on the organization of the testcases
and common* files:
     * The common* scripts have been reorganized into the common/ dir.
     * The testcases have been reorganized into sub test dirs under tests/.
     * The run.* scripts have been removed.

This patch uses the simple way to make install support above changes:
     Make up one Makefile for each newly created subdirs, which can control
     'make install' separately.

v2 introduces the following changes compared with v1:
     * Ignore the file 'group' under the top dir, for it's useless in the new
       structures.
     * Remove the redundant comments in the Makefiles.
     * Use XXX_DIR instead of XXX_SUBDIR in the Makefiles under common/ and tests/.
     * Export TESTS_DIR in teh top level Makefile instead of tests/Makefile.
     * Obtain the names of dirs for testcases by iterating sub dirs under /tests,
       not by enumeration in tests/Makefile.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
[rjohnston@sgi.com Minor modification to Makefile]
Signed-off-by: Rich Johnston <rjohnston@sgi.com>