torture: Provide TMPDIR environment variable to specify tmpdir

Both rcutorture and locktorture currently place temporary files in /tmp,
in keeping with decades-long tradition.  However, sometimes it is useful
to specify an alternative temporary directory, for example, for space
or performance reasons.  This commit therefore causes the torture-test
scripting to use the path specified in the TMPDIR environment variable,
or to fall back to traditional /tmp if this variable is not set.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>