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/var/initialization/workgroup/array/array_i32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,23 @@
+groupshared int zero[2][3];
+
+struct tint_symbol_1 {
+  uint local_invocation_index : SV_GroupIndex;
+};
+
+void main_inner(uint local_invocation_index) {
+  {
+    [loop] for(uint idx = local_invocation_index; (idx < 6u); idx = (idx + 1u)) {
+      const uint i = (idx / 3u);
+      const uint i_1 = (idx % 3u);
+      zero[i][i_1] = 0;
+    }
+  }
+  GroupMemoryBarrierWithGroupSync();
+  int v[2][3] = zero;
+}
+
+[numthreads(1, 1, 1)]
+void main(tint_symbol_1 tint_symbol) {
+  main_inner(tint_symbol.local_invocation_index);
+  return;
+}

test/tint/var/initialization/workgroup/array/i32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,22 @@
+groupshared int zero[3];
+
+struct tint_symbol_1 {
+  uint local_invocation_index : SV_GroupIndex;
+};
+
+void main_inner(uint local_invocation_index) {
+  {
+    [loop] for(uint idx = local_invocation_index; (idx < 3u); idx = (idx + 1u)) {
+      const uint i = idx;
+      zero[i] = 0;
+    }
+  }
+  GroupMemoryBarrierWithGroupSync();
+  int v[3] = zero;
+}
+
+[numthreads(1, 1, 1)]
+void main(tint_symbol_1 tint_symbol) {
+  main_inner(tint_symbol.local_invocation_index);
+  return;
+}