proto: refactor equal_test from explicit table to use makeMessages()

This is important for test coverage: the makeMessages() function is where we
declare the various different test message types. Using this central function is
better than explicitly having to spell out all the various cases in the test
table.

This CL is roughly neutral in terms of lines changed, but actually increases
coverage: in the existing table, not every group of tests had coverage for all
cases (proto2, proto3, editions). This was difficult to spot previously, but now
the different test message types are now clearly spelled out (allTypesNoProto3).

Also, the test now prints the description and message types as subtest name.
This makes it easy to verify coverage by just reading the test output.

Change-Id: I08f541548d087233cbb4686785c456674371c2a3
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/623115
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>