patch 8.2.2587: recover test fails on FreeBSD

Problem:    Recover test fails on FreeBSD.
Solution:   Check for Linux.
diff --git a/src/testdir/test_recover.vim b/src/testdir/test_recover.vim
index 621e315..2867663 100644
--- a/src/testdir/test_recover.vim
+++ b/src/testdir/test_recover.vim
@@ -79,8 +79,9 @@
 endfunc
 
 func Test_nocatch_process_still_running()
-  " assume Unix means sysinfo.uptime can be used
-  CheckUnix
+  " sysinfo.uptime probably only works on Linux
+  CheckLinux
+  " the GUI dialog can't be handled
   CheckNotGui
 
   " don't intercept existing swap file here