libext2fs: improve testing coverage of tst_bitmaps

libext2fs: improve testing coverage of tst_bitmaps

Improve the test coverage of tst_bitmaps by:

   (a) adding the ability to test the legacy (32-bit) bitmap code
   (b) adding tests for ext2fs_find_first_zero_inode_bitmap2() and
       ext2fs_find_first_zero_block_bitmap2()

The recent regressions caused by the addition (and use) of
ext2fs_find_first_zero_inode_bitmap2() would have been caught if we
had added these tests first.  (Another object lesson in why unit tests
are critically important!)

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>