Unit Testing

XCTestCase: base class for unit test suite
    setUp(): run before each individual test
    tearDown(): run after each individual test
    write "test" methods
        accept no parameters
        return no value
        prefixed with "test" in the name
    write "performance test" methods
        use "self.measure { ... }" to mesaure time to execute

XCTAssert...: assertion functions
    XCTAssert: expression yields true
    XCTAssertTrue: boolean condition is true
    XCTAssertFalse: boolean condition is false
    XCTAssertNil: expression yields Nil     XCTAssertEqual: two expressions are equal
    XCTAssertNotEqual: two expressions are inequal
    ... and a few others
    all overloaded to take a trailing "message" argument
        this message is what's displayed when the test fails