Fix test framework to compare strings with embedded nulls

The TEST_EQUIV comparison used strcmp on ast_get_print(), which stops
at the first null byte. This meant any test comparing string literals
with embedded nulls (\x00) would silently pass even if the content
after the null differed — strcmp sees both sides as equal at the
truncation point.

Add a TK_STRING special case in compare_asts that uses ast_name_len
and memcmp for length-aware comparison. This is needed to properly
test compile-time folding of strings containing embedded nulls.

Add StringConcatEmbeddedNull test verifying that "ab\x00cd" + "ef"
folds to "ab\x00cdef" with the correct 7-byte length.

Author: SeanTAllen

test/libponyc/sugar.cc

@@ -2228,3 +2228,22 @@ TEST_F(SugarTest, StringConcatChainMultipleVars)
 
   TEST_EQUIV(short_form, full_form);
 }
+
+
+TEST_F(SugarTest, StringConcatEmbeddedNull)
+{
+  const char* short_form =
+    "class Foo\n"
+    "  var create: U32\n"
+    "  fun f(): String val =>\n"
+    "    \"ab\\x00cd\" + \"ef\"";
+
+  const char* full_form =
+    "use \"builtin\"\n"
+    "class ref Foo\n"
+    "  var create: U32\n"
+    "  fun box f(): String val =>\n"
+    "    \"ab\\x00cdef\"";
+
+  TEST_EQUIV(short_form, full_form);
+}

test/libponyc/util.cc

@@ -155,6 +155,20 @@ static bool compare_asts(ast_t* expected, ast_t* actual, errors_t *errors)
   {
     // Allow expected and actual hygenic names to match.
   }
+  else if(expected_id == TK_STRING)
+  {
+    // Use length-aware comparison for strings so embedded nulls are compared
+    // correctly. strcmp would stop at the first null byte.
+    size_t expected_len = ast_name_len(expected);
+    size_t actual_len = ast_name_len(actual);
+    if(expected_len != actual_len ||
+      memcmp(ast_name(expected), ast_name(actual), expected_len) != 0)
+    {
+      ast_error(errors, expected, "AST string mismatch, got %s, expected %s",
+        ast_get_print(actual), ast_get_print(expected));
+      return false;
+    }
+  }
   else if(strcmp(ast_get_print(expected), ast_get_print(actual)) != 0)
   {
     ast_error(errors, expected, "AST text mismatch, got %s, expected %s",