patch 8.1.0922: terminal scrollback test is flaky

Problem:    Terminal scrollback test is flaky.
Solution:   Wait a bit before running the tail command.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index ba06f36..9529b76 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -311,6 +311,7 @@
   call writefile(range(50), 'Xtext')
   call writefile([
 	\ 'terminal',
+	\ 'sleep 100m',
 	\ 'call feedkeys("tail -n 100 -f Xtext\<CR>", "xt")',
 	\ 'sleep 100m',
 	\ 'call feedkeys("\<C-W>N", "xt")',
@@ -318,21 +319,18 @@
   let buf = RunVimInTerminal('-S XTest_postponed', {})
   " Check that the Xtext lines are displayed and in Terminal-Normal mode
   call term_wait(buf)
-  " TODO: this sometimes fails
-  "call VerifyScreenDump(buf, 'Test_terminal_01', {})
+  call VerifyScreenDump(buf, 'Test_terminal_01', {})
 
   silent !echo 'one more line' >>Xtext
   " Sceen will not change, move cursor to get a different dump
   call term_sendkeys(buf, "k")
   call term_wait(buf)
-  " TODO: this sometimes fails
-  "call VerifyScreenDump(buf, 'Test_terminal_02', {})
+  call VerifyScreenDump(buf, 'Test_terminal_02', {})
 
   " Back to Terminal-Job mode, text will scroll and show the extra line.
   call term_sendkeys(buf, "a")
   call term_wait(buf)
-  " TODO: this sometimes fails
-  "call VerifyScreenDump(buf, 'Test_terminal_03', {})
+  call VerifyScreenDump(buf, 'Test_terminal_03', {})
 
   call term_wait(buf)
   call term_sendkeys(buf, "\<C-C>")