Add more tests to time_test

[datacompboy]

Better test coverage for datetime validation.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[acozzette]

ok to test (this comment will trigger the other test runs)

[datacompboy]

This will cause tests to fail (intentently), as it will succeed only after pull/4147 merged.

[acozzette]

Ok, I figured that might be the case, I'll just merge it as long as that's the only test failure.

[datacompboy]

We see here failure (intended);
[ RUN ] DateTimeTest.WrongDays
Assertion failed: time.day >= 1 && time.day <= (month == 2 && IsLeapYear(year) ? kDaysInMonth[month] + 1 : kDaysInMonth[month]), file C:\projects\protobuf\src\google\protobuf\stubs\time.cc, line 123

It worth re-trigger update after fix merged, to make sure tests get fixed.