patch 8.1.1461: tests do not run or are not reliable on some systems

Problem:    Tests do not run or are not reliable on some systems.
Solution:   Use "findstr" instead of "grep" on MS-Windows. Clear
            PROMPT_COMMAND in the terminal test. Delete temp file. Wait for
            output after executing a debug command. (Yegappan Lakshmanan,
            closes #4479)

src/testdir/test_source.vim

@@ -44,4 +44,5 @@ func Test_source_sandbox()
   call assert_equal('hello', getline(1))
   call assert_fails('sandbox source! Xsourcehello', 'E48:')
   bwipe!
+  call delete('Xsourcehello')
 endfunc