kgdb: 1000 loops for the single step test in kgdbts

The single step test is not terribly costly and it should be able to
pass at 1000 loops successfully in under 1 second.  A non-kgdb timing
regression was found using this test, but it did not occur frequently
because by default the test was only executed a single time.

This patch changes the default for the single step test to 1000
iterations and allows for individual configuration of the single step
test to further exercise the kgdb subsystem when needed.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>