Add mmap tests.

Two test programs are added. The run time is about 1min on my el7
instance.

The test script finishes up with a read/write mmap test on offline
extents to verify the data wait paths in those functions.

One program will perform vfs read/write and mmap read/write calls on
the same file from across 5 threads (mounts) repeatedly.  The goal
is to assure there are no locking issues between read/write paths.

The second test program performs consistency checking on a file that is
repeatedly written/read using memory maps and normal reads and writes,
and the content is verified after every operation.

Signed-off-by: Auke Kok <auke.kok@versity.com>

tests/sequence

@@ -17,6 +17,7 @@ projects.sh
 large-fragmented-free.sh
 format-version-forward-back.sh
 enospc.sh
+mmap.sh
 srch-safe-merge-pos.sh
 srch-basic-functionality.sh
 simple-xattr-unit.sh