Add missing include for ::testing::Test

Fixes google3 build.

Change-Id: I2ff266754f65c7c4cfd00fa4a85e86712266758e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/45581
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: James Price <jrprice@google.com>

src/reader/spirv/parser_impl_test_helper.h

diff --git a/src/reader/spirv/parser_impl_test_helper.h b/src/reader/spirv/parser_impl_test_helper.h
index 0c79ab8..298012f 100644
--- a/src/reader/spirv/parser_impl_test_helper.h
+++ b/src/reader/spirv/parser_impl_test_helper.h
@@ -19,6 +19,7 @@
 #include <string>
 #include <vector>
 
+#include "gtest/gtest.h"
 #include "src/demangler.h"
 #include "src/reader/spirv/parser_impl.h"