[clang-tidy] Fix cert-err33-c to ignore functions with same prefixes as target (llvm#135160)

PR llvm#82952 introduced regex matching for `CheckedFunctions` in
`UnusedReturnValueCheck` which is used by the checker `cert-err33-c`.

Add a testcase and fix false positives by adding end-of-string to target
regex's.

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Signed-off-by: Björn Svensson <[email protected]>

Author: bjosv