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-mat4x3/f32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,16 @@
+cbuffer cbuffer_data : register(b0, space0) {
+  uint4 data[5];
+};
+
+float4x3 tint_symbol_3(uint4 buffer[5], uint offset) {
+  const uint scalar_offset = ((offset + 0u)) / 4;
+  const uint scalar_offset_1 = ((offset + 16u)) / 4;
+  const uint scalar_offset_2 = ((offset + 32u)) / 4;
+  const uint scalar_offset_3 = ((offset + 48u)) / 4;
+  return float4x3(asfloat(buffer[scalar_offset / 4].xyz), asfloat(buffer[scalar_offset_1 / 4].xyz), asfloat(buffer[scalar_offset_2 / 4].xyz), asfloat(buffer[scalar_offset_3 / 4].xyz));
+}
+
+void main() {
+  const float4 x = mul(tint_symbol_3(data, 0u), asfloat(data[4].xyz));
+  return;
+}