(testsuite) Improve `cancel' with dummy print job

A dummy print job is now created (and removed/cancelled) when testing `cancel' completion.
To run the test:

    cd test && ./runCompletion cancel.exp

test/fixture1/foo

@@ -0,0 +1 @@
+foo

test/lib/completions/cancel.exp

@@ -11,7 +11,18 @@ proc teardown {} {
 setup
 
 
-assert_complete_any "cancel "
+    # Adding a print job is successful?
+if {[assert_exec {lp -H hold fixture1/foo} job]} {
+    # Yes, adding a print-job is successful;
+        # Retrieve job-id, so we can cancel the job after the test
+    set job_id [lindex [split $job] 3]
+        # Retrieve list of all current print jobs
+    assert_exec {lpstat | cut -d\040 -f1} jobs
+        # Completion should show all print jobs
+    assert_complete $jobs "cancel "
+        # Remove/cancel the test print job
+    assert_exec "cancel $job_id"
+}; # if
 
 
 sync_after_int