If binary 'bin' has a subtest 'sub', which has dynamic subtests 'foo'
and 'bar', results.json will now have "subtests" by the names
igt@bin@sub@foo and igt@bin@sub@bar, with data as expected of normal
subtests.

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>