Add test failure stack traces to JUnit reports

[buntagonalprism]

Stack traces were previously deliberately excluded from the JUnit output when a test failed an expectation like expect(true, false). Stack traces were only included when an unexpected error was encountered while running the test.

This change includes the stack trace in the JUnit output for test failures, as well as test errors.

An example of where stack traces for test failures may be useful: consider a large application that is using helper methods to verify common behaviour. In this case the failing expectation may not be collocated with the failing test, so having the stack trace is crucial in quickly finding where the issue is occurring.

Also swapped the escaped newline characters in the error for true newlines, which produces a cleaner output.

Fixes #14