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/zero_init/array/bool.wgsl.expected.fxc.hlsl

@@ -0,0 +1,8 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  bool v[4] = (bool[4])0;
+}

test/tint/expressions/zero_init/array/f32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,8 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  float v[4] = (float[4])0;
+}

test/tint/expressions/zero_init/array/i32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,8 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  int v[4] = (int[4])0;
+}

test/tint/expressions/zero_init/array/struct.wgsl.expected.fxc.hlsl

@@ -0,0 +1,15 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+struct S {
+  int i;
+  uint u;
+  float f;
+  bool b;
+};
+
+void f() {
+  S v[4] = (S[4])0;
+}

test/tint/expressions/zero_init/array/u32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,8 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  uint v[4] = (uint[4])0;
+}