cmake: apply parallel build settings to ptest tasks

cmake: apply parallel build settings to ptest tasks

ptest compile and install tasks do not have parallel build settings for
cmake. On powerful build machines this can cause overload situations
and oomkills.
Observed when building qtgrpc with ptest generally enabled in distro.

Having this in ptest class is suboptimal, but creating ptest-cmake class
just for these two variables is probably overkill.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>