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_environ.vim

@@ -36,7 +36,7 @@ func Test_external_env()
 
   call setenv('FOO', v:null)
   if has('win32')
-    let result = system('set | grep ^FOO=')
+    let result = system('set | findstr ^FOO=')
   else
     let result = system('env | grep ^FOO=')
   endif