Add tests for if statement AST

This CL adds tests for the if statement AST node.

Bug: tint:11
Change-Id: Ice3f281e1dfd72d76f43767f7b3040af862f4a58
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16540
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 70cf7b4..d8b608e 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -216,6 +216,7 @@
   ast/fallthrough_statement_test.cc
   ast/function_test.cc
   ast/identifier_expression_test.cc
+  ast/if_statement_test.cc
   ast/import_test.cc
   ast/int_literal_test.cc
   ast/location_decoration_test.cc