Fixture for inspecting the captured output.

Intended to be used with OverloadedRecordDot:

output <- getFixture @FixtureCapturedOutput

-- Read all of stdout/stderr so far
stdout <- output.getStdout
stderr <- output.getStderr

-- Read everything in stdout/stderr since the last read
stdout_chunk <- output.readStdout
stderr_chunk <- output.readStderr