add optional message string to assertXxx() macros

[bxparks]

When assertXxx() is used in a common helper function, or in a for-loop, it would be really nice to be able to add additional message strings into its output, so that we can figure out where the code breakage was more easily. (AUnit doesn't use exceptions, like Google Test and ArduinoUnit, so can't give the stack trace.)

[bxparks]

Related to this, it's now possible (after a recent class hierarchy refactoring) for assertXxx() macros to print out the name of the test in the assertion message. However, in the past, I found that adding an extra parameter to the Assertion::assertion() method consumes several bytes (~10 bytes if I recall). In a large test suite with, say, 300 assertXxx() statements, that increases the flash memory size by 3000 bytes, which is significant for a 32kB microcontroller.

I'll have to do some research to see if it's possible to allow the user to choose to pay for this cost or not.