use sh instead of bash in ProcessUtilTest on linux
use sh instead of bash in ProcessUtilTest on linux

These unit tests don't seem to really depend on bash, by using sh
instead of bash we can run these on more embedded linux targets.

Review URL: https://codereview.chromium.org/1249973002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#339901}

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