patch 9.1.1100: tests: Test_log_nonexistent only works on Linux

Problem:  tests: Test_log_nonexistent only works on Linux
          (after v9.1.1097)
Solution: Add CheckUnix condition

closes: #16476

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_startup.vim b/src/testdir/test_startup.vim
index c16e4ae..78c24e4 100644
--- a/src/testdir/test_startup.vim
+++ b/src/testdir/test_startup.vim
@@ -743,6 +743,7 @@
 func Test_log_nonexistent()
   " this used to crash Vim
   CheckFeature channel
+  CheckUnix
   let result = join(systemlist(GetVimCommand() .. ' --log /X/Xlogfile -c qa!'))
   call assert_match("E484: Can't open file", result)
 endfunc