test: make pgrep call portable
test: make pgrep call portable

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.

MikeMcQuaid/brew,konqui/brew,konqui/brew,nandub/brew,Homebrew/brew,JCount/brew,JCount/brew,vitorgalvao/brew,konqui/brew,sjackman/homebrew,nandub/brew,mahori/brew,vitorgalvao/brew,Homebrew/brew,reitermarkus/brew,vitorgalvao/brew,nandub/brew,claui/brew,JCount/brew,nandub/brew,MikeMcQuaid/brew,konqui/brew,claui/brew,claui/brew,sjackman/homebrew,EricFromCanada/brew,MikeMcQuaid/brew,reitermarkus/brew,reitermarkus/brew,EricFromCanada/brew,claui/brew,Homebrew/brew,Homebrew/brew,JCount/brew,sjackman/homebrew,MikeMcQuaid/brew,EricFromCanada/brew,vitorgalvao/brew,mahori/brew,mahori/brew,mahori/brew,EricFromCanada/brew,sjackman/homebrew,reitermarkus/brew