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/ast/loop_statement_test.cc b/src/ast/loop_statement_test.cc
index 09455da..37788d5 100644
--- a/src/ast/loop_statement_test.cc
+++ b/src/ast/loop_statement_test.cc
@@ -23,6 +23,7 @@
 
 namespace tint {
 namespace ast {
+namespace {
 
 using LoopStatementTest = testing::Test;
 
@@ -183,5 +184,6 @@
 )");
 }
 
+}  // namespace
 }  // namespace ast
 }  // namespace tint