testsuite: Add check for shared memory

Created a test process test_shm that allocates a shared memory
segment and prints the segment ID. pmap testsuite runs pmap to
check that the segment is found.

The value returned by shmget() is the same value that is printed
in the fifth column /proc/<PID>/maps

Signed-off-by: Craig Small <csmall@dropbear.xyz>

lib/.gitignore

@@ -3,5 +3,6 @@
 test_fileutils
 test_nsutils
 test_process
+test_shm
 test_strutils
 test_strtod_nol