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)
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index b97cdb3..9d6fddd 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -8,6 +8,7 @@
 source screendump.vim
 
 let s:python = PythonProg()
+let $PROMPT_COMMAND=''
 
 " Open a terminal with a shell, assign the job to g:job and return the buffer
 " number.