misc: add duration for long soak tests

misc: add duration for long soak tests

Make it so that test runners can schedule long soak stress test programs
for an exact number of seconds by setting the SOAK_DURATION config
variable.  Change the definition of the 'soak' test to specify that
these tests can be controlled via SOAK_DURATION.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Andrey Albershteyn <aalbersh@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>