Add file-sniffer test that fails for bitstrings

[angelikatyborska]

I'm currently thinking through this exercise because of the analyzer issues, and I realized that we could use one more test. This new unit test will catch two problems:

• people using String.starts_with? (it crashes for non-binaries)
• people using ::bistring instead of ::binary (I saw a few of those in community solutions)

[github-actions]

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[jiegillet]

Good ideas!