patch 8.1.1638: running tests leaves some files behind

Problem:    Running tests leaves some files behind.
Solution:   Delete the files. (Ozaki Kiichi, closes #4617)
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index a08cef7..9199ba7 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -1528,10 +1528,10 @@
   call assert_equal([''], getbufline(buf, 1, '$'))
 
   let curbuf = bufnr('')
-  call writefile(['some', 'text'], 'otherName')
-  let buf = bufadd('otherName')
+  call writefile(['some', 'text'], 'XotherName')
+  let buf = bufadd('XotherName')
   call assert_notequal(0, buf)
-  call assert_equal(1, bufexists('otherName'))
+  call assert_equal(1, bufexists('XotherName'))
   call assert_equal(0, getbufvar(buf, '&buflisted'))
   call assert_equal(0, bufloaded(buf))
   call bufload(buf)
@@ -1554,6 +1554,7 @@
   call assert_equal(0, bufexists(buf2))
 
   bwipe someName
-  bwipe otherName
+  bwipe XotherName
   call assert_equal(0, bufexists('someName'))
+  call delete('XotherName')
 endfunc