patch 8.2.0710: Netbeans test sometimes fails

Problem:    Netbeans test sometimes fails.
Solution:   Mark any test using an external command as flaky.

src/testdir/shared.vim

@@ -36,6 +36,9 @@ endfunc
 
 " Run "cmd".  Returns the job if using a job.
 func RunCommand(cmd)
+  " Running an external command can occasionally be slow or fail.
+  let g:test_is_flaky = 1
+
   let job = 0
   if has('job')
     let job = job_start(a:cmd, {"stoponexit": "hup"})