rszshm: disable valgrind for tests.

It returns EINVAL instead of ENOMEM for test/run.c line 96, then
complains on line 137:

==29368== Invalid read of size 4
==29368==    at 0x4033BC: main (run.c:137)
==29368==  Address 0x400000000018 is not stack'd, malloc'd or (recently) free'd

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>