vim-patch:8.2.1267: MS-Windows: tests may fail due to $PROMPT value

Problem:    MS-Windows: tests may fail due to $PROMPT value.
Solution:   Set $PROMPT for testing. (Taro Muraoka, closes vim/vim#6510)
2690b5aed8

src/nvim/testdir/runtest.vim

@@ -84,6 +84,11 @@ let &runtimepath .= ','.expand($BUILD_DIR).'/runtime/'
 " Always use forward slashes.
 set shellslash
 
+if has('win32')
+  " avoid prompt that is long or contains a line break
+  let $PROMPT = '$P$G'
+endif
+
 " Prepare for calling test_garbagecollect_now().
 let v:testing = 1