test: cleanup NFS test case

Drop the disabling of kdump.service, there is hardly any test usage for
it as on Fedora it's not enabled and on RHEL it is and running simply
disabling does not do anything for us.

Drop the removal of /var/crash, enableKdump used to re-create it anyway.
Test if the dir is empty like TestKdump.testBasic does.

test/verify/check-kdump

@@ -28,7 +28,6 @@ class KdumpHelpers(testlib.MachineCase):
             self.machine.execute("grubby --args=crashkernel=192M --update-kernel=ALL")
             self.machine.reboot()
 
-        self.machine.execute("mkdir -p /var/crash")
         self.allow_restart_journal_messages()
         self.machine.start_cockpit()
         self.browser.switch_to_top()
@@ -421,8 +420,10 @@ class TestKdumpNFS(KdumpHelpers):
         self.machines["nfs"].write("/etc/exports", "/srv/kdump 10.111.113.0/24(rw,no_root_squash)\n")
         self.machines["nfs"].execute("mkdir -p /srv/kdump/var/crash; firewall-cmd --add-service nfs; systemctl restart nfs-server")
 
-        # ensure there is no local /var/crash, should not break kdump
-        m.execute("rmdir /var/crash")
+        # there shouldn't be any crash reports in the target directory
+        self.assertEqual(m.execute("find /var/crash -maxdepth 1 -mindepth 1 -type d"), "")
+
+        # set up client machine
         self.login_and_go("/kdump")
         self.enableKdump()