test: make pgrep call portable

[iMichka]

Fixes (on linux), for example for the ninja formula test:

pgrep
-P
83252
{out: :close}}
Invalid preceding regular expressionpgrep: write error: Bad file descriptor
Error: ninja: failed
An exception occurred within a child process:
  BuildError: Failed executing: pgrep -P 83252 {out: :close}

Using File:NULL is more portable.

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[MikeMcQuaid]

Nice work @iMichka!