Move tests into anonymous namespace.

This CL moves all test files to the anonymous namespace.

Change-Id: Ie5ae0efe907c03eeb441e90f31c976844172b594
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/17900
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
diff --git a/src/reader/wgsl/parser_impl_and_expression_test.cc b/src/reader/wgsl/parser_impl_and_expression_test.cc
index 262611c..dac8bc9 100644
--- a/src/reader/wgsl/parser_impl_and_expression_test.cc
+++ b/src/reader/wgsl/parser_impl_and_expression_test.cc
@@ -23,6 +23,7 @@
 namespace tint {
 namespace reader {
 namespace wgsl {
+namespace {
 
 TEST_F(ParserImplTest, AndExpression_Parses) {
   auto p = parser("a & true");
@@ -69,6 +70,7 @@
   ASSERT_TRUE(e->IsIdentifier());
 }
 
+}  // namespace
 }  // namespace wgsl
 }  // namespace reader
 }  // namespace tint