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/statements/switch/common.wgsl.expected.fxc.hlsl

@@ -0,0 +1,24 @@
+[numthreads(1, 1, 1)]
+void f() {
+  int i = 0;
+  int result = 0;
+  switch(i) {
+    case 0: {
+      result = 10;
+      break;
+    }
+    case 1: {
+      result = 22;
+      break;
+    }
+    case 2: {
+      result = 33;
+      break;
+    }
+    default: {
+      result = 44;
+      break;
+    }
+  }
+  return;
+}

test/tint/statements/switch/fallthrough.wgsl.expected.fxc.hlsl

@@ -0,0 +1,16 @@
+[numthreads(1, 1, 1)]
+void f() {
+  int i = 0;
+  switch(i) {
+    case 0: {
+      /* fallthrough */
+      {
+        break;
+      }
+    }
+    default: {
+      break;
+    }
+  }
+  return;
+}

test/tint/statements/switch/only_default_case.wgsl.expected.fxc.hlsl

@@ -0,0 +1,11 @@
+[numthreads(1, 1, 1)]
+void f() {
+  int i = 0;
+  int result = 0;
+  i;
+  do {
+    result = 44;
+    break;
+  } while (false);
+  return;
+}