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/expressions/binary/mul/vec3-scalar/f32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,6 @@
+[numthreads(1, 1, 1)]
+void f() {
+  const float3 a = float3(1.0f, 2.0f, 3.0f);
+  const float3 r = (a * 4.0f);
+  return;
+}

test/tint/expressions/binary/mul/vec3-scalar/i32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,6 @@
+[numthreads(1, 1, 1)]
+void f() {
+  const int3 a = int3(1, 2, 3);
+  const int3 r = (a * 4);
+  return;
+}

test/tint/expressions/binary/mul/vec3-scalar/u32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,6 @@
+[numthreads(1, 1, 1)]
+void f() {
+  const uint3 a = uint3(1u, 2u, 3u);
+  const uint3 r = (a * 4u);
+  return;
+}