tests: Replace wc -l with grep -c

[casperklein]

Description

grep
-c, --count
Suppress normal output; instead print a count of matching lines for each input file.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added information about changes made in this PR to CHANGELOG.md

[polarathene]

Could we use --count? 😅

Either is fine, just makes it a little more obvious (I'm terrible at remembering short flags I don't use often).

[casperklein]

I used the short form, to match the existing style of the other (short) arguments.