Initial test constructor for FFT tests
Two new functions to write tests in fast-fourier-transforms/example.lisp, #'make-real-noise-fft-test and #'generate-fft-tests. The first one generates, given a size and stride, a pair of forward and inverse real tests, together with the answers. The second one loops over a range of sizes and strides, collecting the sets, and wrapping with a lisp-unit:define-test. Since they rely on #'make-test in test-unit/generate-tests.lisp, they won't normally be compiled. No test file generated yet.