Don't mutate fn values for functions entirely composed of skipped statements

[sourcefrog]

cargo-mutate reports that a testing utility function I have doesn't affect anything when mutated from a series of assert! macros into (). Maybe it would be possible to ignore this mutation if all of the statements are assert!-family macros (though itertools::assert_equal is in the same boat, just not an official macro).

Originally posted by @mathstuf in #301 (comment)