make megatest consistent with unjoined tests wrt newlines, honor newlines in output spec (#16151)

* fix megatest newlines
* still allow missing trailing newline for now but in a more strict way than before

tests/exception/texcas.nim

@@ -1,8 +1,9 @@
 discard """
   targets: "c cpp"
-  output: '''Hello
+  output: '''
 Hello
-  '''
+Hello
+'''
 """
 proc test[T]() =
   try:

tests/exception/texceptions.nim

@@ -1,5 +1,6 @@
 discard """
   output: '''
+
 BEFORE
 FINALLY