tint/test-runner: Split expectations for FXC and DXC

Change tint's `--fxc` flag to take the path of the FXC compiler DLL.
Have tint attempt to validate with both FXC and DXC if `--validate` is
passed.

Fix the 'dirsWithNoPassExpectations' logic which looks like it got
broken with the tint -> dawn merge. It also incorrectly applied
filepath.FromSlash() on windows.

Change-Id: I0f46aa5c21bc48a2abc48402c41f846aff4a8633
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/96800
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>

test/tint/ptr_ref/load/param/ptr.spvasm.expected.fxc.hlsl

@@ -0,0 +1,17 @@
+int func(int value, inout int pointer) {
+  const int x_9 = pointer;
+  return (value + x_9);
+}
+
+void main_1() {
+  int i = 0;
+  i = 123;
+  const int x_18 = func(i, i);
+  return;
+}
+
+[numthreads(1, 1, 1)]
+void main() {
+  main_1();
+  return;
+}

test/tint/ptr_ref/load/param/ptr.wgsl.expected.fxc.hlsl

@@ -0,0 +1,10 @@
+int func(int value, inout int pointer) {
+  return (value + pointer);
+}
+
+[numthreads(1, 1, 1)]
+void main() {
+  int i = 123;
+  const int r = func(i, i);
+  return;
+}