test: Generate expected output for all tests

The expected output is far from perfect, and the generated HLSL and MSL
isn't even validated yet, so may be incorrect.

However, by committing the generated output, we get clear examples of
the currently generated output of each backend. As we land fixes and
improvements to each backend, the presubmits will require us to update
the expected test output, and so code reviews will include diffs of
each backend's generated output.

Change-Id: I5c2a9e5b796d0ab75b3ec4c7f8ad00a0a2ab166f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51224
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: David Neto <dneto@google.com>

test/samples/function.wgsl.expected.wgsl

@@ -0,0 +1,7 @@
+fn main() -> f32 {
+  return (((2.0 * 3.0) - 4.0) / 5.0);
+}
+
+[[stage(compute), workgroup_size(2, 1, 1)]]
+fn ep() {
+}