patch 8.2.0390: terminal postponed scrollback test is flaky

Problem:    Terminal postponed scrollback test is flaky.
Solution:   Add delay in between sending keys.  Rename dump files.

src/testdir/test_terminal.vim

@@ -352,22 +352,24 @@ func Test_terminal_postponed_scrollback()
 	\ ], 'XTest_postponed')
   let buf = RunVimInTerminal('-S XTest_postponed', {})
   " Check that the Xtext lines are displayed and in Terminal-Normal mode
-  call VerifyScreenDump(buf, 'Test_terminal_01', {})
+  call VerifyScreenDump(buf, 'Test_terminal_scrollback_1', {})
 
   silent !echo 'one more line' >>Xtext
   " Screen will not change, move cursor to get a different dump
   call term_sendkeys(buf, "k")
-  call VerifyScreenDump(buf, 'Test_terminal_02', {})
+  call VerifyScreenDump(buf, 'Test_terminal_scrollback_2', {})
 
   " Back to Terminal-Job mode, text will scroll and show the extra line.
   call term_sendkeys(buf, "a")
-  call VerifyScreenDump(buf, 'Test_terminal_03', {})
+  call VerifyScreenDump(buf, 'Test_terminal_scrollback_3', {})
 
-  call term_wait(buf)
+  " stop "tail -f"
   call term_sendkeys(buf, "\<C-C>")
-  call term_wait(buf)
+  call term_wait(buf, 50)
+  " stop shell
   call term_sendkeys(buf, "exit\<CR>")
-  call term_wait(buf)
+  call term_wait(buf, 100)
+  " close terminal window
   let tsk_ret = term_sendkeys(buf, ":q\<CR>")
 
   " check type of term_sendkeys() return value

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    390,
 /**/
     389,
 /**/