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/extensions/parsing/duplicated_extensions.wgsl.expected.dxc.hlsl

@@ -0,0 +1,14 @@
+struct tint_symbol {
+  float4 value : SV_Target0;
+};
+
+float4 main_inner() {
+  return float4(0.100000001f, 0.200000003f, 0.300000012f, 0.400000006f);
+}
+
+tint_symbol main() {
+  const float4 inner_result = main_inner();
+  tint_symbol wrapper_result = (tint_symbol)0;
+  wrapper_result.value = inner_result;
+  return wrapper_result;
+}

test/tint/extensions/parsing/duplicated_extensions.wgsl.expected.fxc.hlsl

@@ -0,0 +1,14 @@
+struct tint_symbol {
+  float4 value : SV_Target0;
+};
+
+float4 main_inner() {
+  return float4(0.100000001f, 0.200000003f, 0.300000012f, 0.400000006f);
+}
+
+tint_symbol main() {
+  const float4 inner_result = main_inner();
+  tint_symbol wrapper_result = (tint_symbol)0;
+  wrapper_result.value = inner_result;
+  return wrapper_result;
+}