ptest.bbclass: Honor PARALLEL_MAKE, PARALLEL_MAKEINST

ptest.bbclass: Honor PARALLEL_MAKE, PARALLEL_MAKEINST

Since oe_runmake wrapper doesn't by default involve PARALLEL_MAKE
outside do_compile() nor PARALLEL_MAKEINST outside do_install(),
enable parallellized make by default when oe_runmake is invoked from
do_compile_ptest() or do_install_ptest() by declaring wrapper task
specific EXTRA_OEMAKE overrides in fashion similar to do_compile and
do_install overrides in meta/conf/bitbake.conf.

Parallel make can still be disabled by resetting bbclass specific
PTEST_PARALLEL_MAKE and PTEST_PARALLEL_MAKEINST variables in recipe
e.g. if a race issue needs to be avoided without modifying source code.

Tested by issuing following command sequence on a 32-core build host:

  $ bitbake -c clean util-linux && bitbake --skip-setscene -c compile util-linux && time bitbake --skip-setscene -c compile_ptest_base util-linux

and found that before this change the result was

  real	0m34.684s
  user	0m0.753s
  sys	0m0.131s

and after this change

  real	0m9.868s
  user	0m0.749s
  sys	0m0.150s

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>