Move msl_test.cc to correct source_set

Should fix dawn's autorollers

Change-Id: I6d46af611a3fc699eabeec92b395d9df48cc03fa
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/44040
Commit-Queue: Ben Clayton <bclayton@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 3e4e0ae..e2c107e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -973,7 +973,6 @@
 
 source_set("tint_unittests_spv_writer_src") {
   sources = [
-    "src/transform/msl_test.cc",
     "src/transform/spirv_test.cc",
     "src/writer/spirv/binary_writer_test.cc",
     "src/writer/spirv/builder_accessor_expression_test.cc",
@@ -1180,6 +1179,7 @@
 
 source_set("tint_unittests_msl_writer_src") {
   sources = [
+    "src/transform/msl_test.cc",
     "src/writer/msl/generator_impl_alias_type_test.cc",
     "src/writer/msl/generator_impl_array_accessor_test.cc",
     "src/writer/msl/generator_impl_assign_test.cc",