add test

Author: maorwayn

checker/src/test/resources/standardEnvDump.baseline

@@ -256,7 +256,7 @@ declare string {
   function int64_to_string (int) -> string
   function uint64_to_string (uint) -> string
   function double_to_string (double) -> string
-  function bool_to_string (double) -> string
+  function bool_to_string (bool) -> string
   function bytes_to_string (bytes) -> string
   function timestamp_to_string (google.protobuf.Timestamp) -> string
   function duration_to_string (google.protobuf.Duration) -> string

runtime/src/test/resources/stringConversions.baseline

@@ -13,6 +13,11 @@ Source: string(-1)
 bindings: {}
 result:   -1
 
+Source: string(true)
+=====>
+bindings: {}
+result:   true
+
 Source: string(b'abc\303\203')
 =====>
 bindings: {}

testing/src/main/java/dev/cel/testing/BaseInterpreterTest.java

@@ -1657,6 +1657,9 @@ public void stringConversions() {
     source = "string(-1)"; // int converts to '-1'
     runTest();
 
+    source = "string(true)"; // bool converts to 'true'
+    runTest();
+
     // Byte literals in Google SQL only take the leading byte of an escape character.
     // This means that to translate a byte literal to a UTF-8 encoded string, all bytes must be
     // encoded in the literal as they would be laid out in memory for UTF-8, hence the extra octal