fix multiline string tests

tests/test_cases/input/multiline_string_whitespace_only.kdl

@@ -1,18 +1,19 @@
 // This file deliberately contains unusual whitespace
-// The first two strings are empty
+// The first three strings are empty, because whitespace-only lines collapse to
+// just `\n`.
 node """
   	""" """
  	 \
-             
+             
  	 """ """
-                            
+                             
  """\
     \ // The next two strings contains only whitespace
     """
        
-       
+           
       \s 
     """ #"""
-    
+      
 
   """#

tests/test_cases/input/multiline_string_whitespace_only_non_matching_prefix_fail.kdl

@@ -0,0 +1,19 @@
+// This file deliberately contains unusual whitespace
+// Parsing should fail because not all strings have a matching multiline prefix,
+// which must be exact, down to the codepoint, _before_ newline normalization.
+node """
+  	""" """
+ 	 \
+             
+ 	 """ """
+                            
+ """\
+    \ // The next two strings contains only whitespace
+    """
+       
+       
+      \s 
+    """ #"""
+    
+
+  """#