[wpilib] Add epoch print tests to Tracer

[calcmogul]

Verifying the contents of epoch prints would have caught #2392.

Here's an example of verifying stdout contents with Google Test: https://github.com/wpilibsuite/allwpilib/blob/master/wpiutil/src/test/native/cpp/UnitsTest.cpp#L1307-L1310

Here's an example of verifying stdout contents with JUnit: https://stackoverflow.com/a/1119559/5484466

[Starlight220]

I can do Java, how do I get output from DS.reportWarning()?

[calcmogul]

In unit tests, ReportWarning() just prints to stderr, so you just need to capture console output.

[Starlight220]

In Java, there is an overload that takes a lambda - can be used for testing. Dunno if C++ has this option as well.

[Starlight220]

Would it be good enough to make sure that the output conforms to a certain regex, or is something more thorough required?

[AustinShalit]

For Java, you might take a look to see if SystemRules was updated for JUnit 5: stefanbirkner/system-rules#55