drm/i915/selftests: Tweak the default subtest runtime

BAT is growing a little fat and CI is under pressure and needs to trim
off some redundant runtime. An easy option is to reduce the selftest
runtimes, so try halving our default subtest timeout. While this reduces
the number of iterations used, for the majority of tests that are
passing, repeat runs (with different CI_DRM) will make up the
difference -- a negative consequence though is that we may reduce the
frequency of sporadic failures. Hopefully, we have no tests that were
crucially dependent on the previous 1s timeout...

Suggested-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025092749.13468-1-chris@chris-wilson.co.uk