patch 9.0.0719: too many delete() calls in tests

Problem:    Too many delete() calls in tests.
Solution:   Use deferred delete where possible.
diff --git a/src/testdir/test_signals.vim b/src/testdir/test_signals.vim
index ea25ae1..c37ea2a 100644
--- a/src/testdir/test_signals.vim
+++ b/src/testdir/test_signals.vim
@@ -117,7 +117,7 @@
     au VimSuspend * call writefile(["VimSuspend triggered"], "XautoOut1", "as")
     au VimResume * call writefile(["VimResume triggered"], "XautoOut2", "as")
   END
-  call writefile(lines, 'XsetupAucmd')
+  call writefile(lines, 'XsetupAucmd', 'D')
 
   let buf = RunVimInTerminal('-S XsetupAucmd Xsig_TERM', {'rows': 6})
   let pid_vim = term_getjob(buf)->job_info().process
@@ -147,7 +147,6 @@
 
   %bwipe!
   call delete('.Xsig_TERM.swp')
-  call delete('XsetupAucmd')
   call delete('XautoOut1')
   call delete('XautoOut2')
 endfunc
@@ -176,7 +175,7 @@
     au VimLeave * call writefile(["VimLeave triggered"], "XautoOut", "as")
     au VimLeavePre * call writefile(["VimLeavePre triggered"], "XautoOut", "as")
   END
-  call writefile(lines, 'XsetupAucmd')
+  call writefile(lines, 'XsetupAucmd', 'D')
 
   let buf = RunVimInTerminal('-S XsetupAucmd Xsig_TERM', {'rows': 6})
   let pid_vim = term_getjob(buf)->job_info().process
@@ -200,7 +199,6 @@
 
   %bwipe!
   call delete('.Xsig_TERM.swp')
-  call delete('XsetupAucmd')
   call delete('XautoOut')
 endfunc