patch 8.2.0305: relativenumber test fails on some systems

Problem:    Relativenumber test fails on some systems. (James McCoy)
Solution:   Clear the command line.
diff --git a/src/testdir/test_number.vim b/src/testdir/test_number.vim
index 5417585..fef187b 100644
--- a/src/testdir/test_number.vim
+++ b/src/testdir/test_number.vim
@@ -284,10 +284,10 @@
   " Default colors
   call VerifyScreenDump(buf, 'Test_relnr_colors_1', {})
 
-  call term_sendkeys(buf, ":hi LineNrAbove ctermfg=blue\<CR>")
+  call term_sendkeys(buf, ":hi LineNrAbove ctermfg=blue\<CR>:\<CR>")
   call VerifyScreenDump(buf, 'Test_relnr_colors_2', {})
 
-  call term_sendkeys(buf, ":hi LineNrBelow ctermfg=green\<CR>")
+  call term_sendkeys(buf, ":hi LineNrBelow ctermfg=green\<CR>:\<CR>")
   call VerifyScreenDump(buf, 'Test_relnr_colors_3', {})
 
   call term_sendkeys(buf, ":hi clear LineNrAbove\<CR>")