Move the test inputs for the wire marshaler and unmarshaler out of
decode_test.go and into a new file. Consolidate some tests for invalid
messages (UTF-8 validation failures, field numbers out of range) into
a single list of invalid messages. Break out the no-enforce-utf8 test
into a separate file, since it is both complicated and conditional on
legacy support.
Change-Id: Ide80fa9d3aec2b6d42a57e6f9265358aa5e661a7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/211557
Reviewed-by: Joe Tsai <joetsai@google.com>
