Enable parallel test execution with grm
Enable parallel test execution with grm

mbedgt creats a dummy device when using grm.
When the --parallel option is enabled, there
is still only one dummy device, hence parallel
test execution does not work.

This fix add multiple dummy devices according
to the number specified by the --parallel option.
Thus enable test execution in parallel.

Limitation:
mbedgt still does not know how many devices are
actually available in the grm, hence the burden
of making sure there are available devices falls
on the user.

ARMmbed/greentea