patch 8.0.1249: no error when WaitFor() gets an invalid wrong expression

Problem:    No error when WaitFor() gets an invalid wrong expression.
Solution:   Do not ignore errors in evaluationg the expression.  Fix places
            where the expression was wrong.
diff --git a/src/testdir/test_netbeans.vim b/src/testdir/test_netbeans.vim
index 4ac03db..aa4e676 100644
--- a/src/testdir/test_netbeans.vim
+++ b/src/testdir/test_netbeans.vim
@@ -19,6 +19,7 @@
 
 func Nb_basic(port)
   call delete("Xnetbeans")
+  call writefile([], "Xnetbeans")
   exe 'nbstart :localhost:' . a:port . ':bunny'
   call assert_true(has("netbeans_enabled"))
 
@@ -53,6 +54,9 @@
 endfunc
 
 func Nb_file_auth(port)
+  call delete("Xnetbeans")
+  call writefile([], "Xnetbeans")
+
   call assert_fails('nbstart =notexist', 'E660:')
   call writefile(['host=localhost', 'port=' . a:port, 'auth=bunny'], 'Xnbauth')
   if has('unix')