Fix string literal comparison errors in bob_test.cpp

[mgrubb]

When comparing against string literals with == operator compiler throws /Users/mgrubb/Exercism/cpp/bob/bob_test.cpp:15:25: error: result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Werror,-Wstring-compare] REQUIRE("Whatever." == bob::hey("Tom-ay-to, tom-aaaah-to."));

This prevents the tests from passing.
This change creates std::string constants to compare against.

[github-actions]

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[siebenschlaefer]

Please discuss contributions like this in the forum first.

May I ask: What is the return type of hey in your function? It's probably not std::string nor std::string_view.