patch 9.0.1535: test commented out in a wrong way

Problem:    Test commented out in a wrong way.
Solution:   Use legacy script comment character.
diff --git a/src/testdir/test_substitute.vim b/src/testdir/test_substitute.vim
index 73e6bef..3565c53 100644
--- a/src/testdir/test_substitute.vim
+++ b/src/testdir/test_substitute.vim
@@ -1416,18 +1416,18 @@
 
 " Check handling expanding "~" resulting in extremely long text.
 " FIXME: disabled, it takes too long to run on CI
-#func Test_substitute_tilde_too_long()
-#  enew!
-#
-#  s/.*/ixxx
-#  s//~~~~~~~~~AAAAAAA@(
-#
-#  " Either fails with "out of memory" or "text too long".
-#  " This can take a long time.
-#  call assert_fails('sil! norm &&&&&&&&&', ['E1240:\|E342:'])
-#
-#  bwipe!
-#endfunc
+"func Test_substitute_tilde_too_long()
+"  enew!
+"
+"  s/.*/ixxx
+"  s//~~~~~~~~~AAAAAAA@(
+"
+"  " Either fails with "out of memory" or "text too long".
+"  " This can take a long time.
+"  call assert_fails('sil! norm &&&&&&&&&', ['E1240:\|E342:'])
+"
+"  bwipe!
+"endfunc
 
 " This should be done last to reveal a memory leak when vim_regsub_both() is
 " called to evaluate an expression but it is not used in a second call.