Fix error in unmarked test script

[dpanici]

the head -n -2 was incorrect (-2 should just be 2), the script was silently failing locally for me (on macOS zsh) and causing it to not actually detect any unmarked tests even if they were present.

[f0uriest]

Are there any tests we forgot to mark that weren't caught?

[YigitElma]

Are there any tests we forgot to mark that weren't caught?

I ran bash devtools/check_unmarked_tests.sh tests/ locally and couldn't find any missing tests.

[YigitElma]

On my system, both versions of the script can catch the missing tests (if I delete some existing ones), this might be OS dependent maybe?

[dpanici]

Are there any tests we forgot to mark that weren't caught?

Not that I saw

On my system, both versions of the script can catch the missing tests (if I delete some existing ones), this might be OS dependent maybe?

Hm I ran on macOS. But if the version in this PR works for both linux and macOS I'd prefer this version