Add case statement tests

This Cl adds tests for the case statement class.

Change-Id: I585fdff9631ed44e6fb297328ed164297f5fa8f3
Bug: tint:11
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16421
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 0434df8..374e44a 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -201,6 +201,7 @@
   ast/break_statement_test.cc
   ast/builtin_decoration_test.cc
   ast/call_expression_test.cc
+  ast/case_statement_test.cc
   ast/entry_point_test.cc
   ast/import_test.cc
   ast/int_literal_test.cc