Add unit-tests using Xemu's testing framework

[mlund]

This enables tests using Xemu but could be expanded to also send to the serial monitor on real hardware. Currently enabled for clang via the cmake and ctest. An example output looks like this:

$ make tests
Test project /Users/mikael/github/mega65-libc/build
    Start 1: test-memory
1/1 Test #1: test-memory ......................   Passed    0.84 sec

100% tests passed, 0 tests failed out of 1

Total Test time (real) =   0.84 sec

Closes #24.