Work around an OpenBSD "make" quirk

Work around an OpenBSD "make" quirk

Consider the following Makefile:

    foo:
    	false

On OpenBSD, the following happens for this Makefile:

  - "make foo" returns 1,
  - "make -k foo" returns 0,
  - "make -k -j6 foo" returns 1.

However, if the .NOTPARALLEL pseudo-target is added to this Makefile,
"make -k -j6 foo" will return 0 as well.

Since bin/tests/Makefile contains the .NOTPARALLEL pseudo-target,
running "make -k -j6 test" from bin/tests/ on OpenBSD prevents any
errors from being reported through that command's exit code.

Work around the issue by running "make -k -j6 test" in the
bin/tests/system/ directory instead as bin/tests/system/Makefile does
not contain the .NOTPARALLEL pseudo-target and thus things work as
expected there.