For a common case of events being simple struct types with some exported
fields, add a helper to check (reflectively) for equal values using cmp.Diff so
that a failed comparison gives a useful diff in the test output.
More complex uses will still want to provide their own comparisons; this
(intentionally) does not export diff options or other hooks from the cmp
package.
Updates #15160
Change-Id: I86bee1771cad7debd9e3491aa6713afe6fd577a6
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>
6992f958fc