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/array/function_parameter.wgsl.expected.fxc.hlsl

@@ -0,0 +1,22 @@
+float f1(float a[4]) {
+  return a[3];
+}
+
+float f2(float a[3][4]) {
+  return a[2][3];
+}
+
+float f3(float a[2][3][4]) {
+  return a[1][2][3];
+}
+
+[numthreads(1, 1, 1)]
+void main() {
+  const float a1[4] = (float[4])0;
+  const float a2[3][4] = (float[3][4])0;
+  const float a3[2][3][4] = (float[2][3][4])0;
+  const float v1 = f1(a1);
+  const float v2 = f2(a2);
+  const float v3 = f3(a3);
+  return;
+}