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/private/array/array_i32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,9 @@
+static int zero[2][3] = (int[2][3])0;
+static int init[2][3] = {{1, 2, 3}, {4, 5, 6}};
+
+[numthreads(1, 1, 1)]
+void main() {
+  int v0[2][3] = zero;
+  int v1[2][3] = init;
+  return;
+}

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

@@ -0,0 +1,9 @@
+static int zero[3] = (int[3])0;
+static int init[3] = {1, 2, 3};
+
+[numthreads(1, 1, 1)]
+void main() {
+  int v0[3] = zero;
+  int v1[3] = init;
+  return;
+}