report error magnitude on @test x ≈ y failure

[stevengj]

As commented on discourse, it would be nice to update the @test macro to report the absolute and relative errors on a failure of an x ≈ y test.

We already specialize the test output on failed comparisons to report the operands:

julia> x, y = 1, 1 + 1e-5;

julia> @test x ≈ y
Test Failed at REPL[63]:1
  Expression: x ≈ y
   Evaluated: 1 ≈ 1.00001

but it would be good to show the absolute error norm(x - y) and/or the relative error norm(x - y) / max(norm(x), norm(y)) e.g.

julia> @test x ≈ y
Test Failed at REPL[63]:1
  Expression: x ≈ y
   Evaluated: 1 ≈ 1.00001
    (absolute error 1e-05 exceeds atol=0,
     relative error 1e-05 exceeds rtol=1.49e-08)

(where it should suffice to print the errors with %0.3g precision.) Or maybe only print the error that failed (i.e. either the atol or rtol, whichever is bigger).