strain: update test output formatting
strain: update test output formatting

see #414
replace `%v` with `%#v` to provide a clearer output in the case of a
failed test. `%#v` is used rather than `%q` as suggested in #414 as it
provides a more accurate output, Strings[a, b, c] appears slightly
confusing.

strain.Strings{"apple", "zebra", "banana", "zombies", "cherimoya", "zelot"}.Keep()

rather than:

Strings[apple zebra banana zombies cherimoya zelot].Keep()

exercism/xgo,leenipper/xgo,leenipper/xgo,robphoenix/exercism-go,exercism/xgo,robphoenix/exercism-go