Add initial unit tests

- Add µnit source and header files
- Add test target to the build system
- Implement a thorough lexer test suite
- Implement a minimal AST test suite

tests/.clang-format

@@ -0,0 +1,6 @@
+BasedOnStyle:    LLVM
+IndentWidth:     4
+Cpp11BracedListStyle: true
+AlignArrayOfStructures: Left
+AllowShortFunctionsOnASingleLine: Empty
+ColumnLimit: 120