patch 8.2.1304: debug backtrace isn't tested much

Problem:    Debug backtrace isn't tested much.
Solution:   Add more specific tests. (Ben Jackson, closes #6540)
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 3cf2a50..6cc7202 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -13,6 +13,9 @@
 " For csh:
 "     setenv TEST_FILTER Test_channel
 "
+" While working on a test you can make $TEST_NO_RETRY non-empty to not retry:
+"     export TEST_NO_RETRY=yes
+"
 " To ignore failure for tests that are known to fail in a certain environment,
 " set $TEST_MAY_FAIL to a comma separated list of function names.  E.g. for
 " sh/bash:
@@ -440,9 +443,11 @@
   call RunTheTest(g:testfunc)
 
   " Repeat a flaky test.  Give up when:
+  " - $TEST_NO_RETRY is not empty
   " - it fails again with the same message
   " - it fails five times (with a different message)
   if len(v:errors) > 0
+        \ $TEST_NO_RETRY == ''
         \ && (index(s:flaky_tests, g:testfunc) >= 0
         \      || g:test_is_flaky)
     while 1