kola/harness.go: fix kola panic

Recently we have seen non-exclusive tests panicking due to
runtime.Goexit executing, but somehow harness.finished being false.
This problem was occurring because we pass tcluster to collectLogsExternalTest
instead of newTC. tcluster has a reference to the harness of the wrapper test,
thus harness.finished is incorrectly set to true for the wrapper test's harness
instead of the non-exclusive subtest's harness.

closes: coreos#2701

mantle/kola/harness.go

@@ -1276,17 +1276,23 @@ func makeNonExclusiveTest(bucket int, tests []*register.Test, flight platform.Fl
 				t := t
 				run := func(h *harness.H) {
 					testResults.add(h)
+					// tcluster has a reference to the wrapper's harness
+					// We need a new TestCluster that has a reference to the
+					// subtest being ran
+					// This will allow timeout logic to work correctly when executing
+					// functions such as TestCluster.SSH, since these functions
+					// internally use harness.RunWithExecTimeoutCheck
+					newTC := cluster.TestCluster{
+						H:       h,
+						Cluster: tcluster.Cluster,
+					}
 					// Install external test executable
 					if t.ExternalTest != "" {
-						setupExternalTest(h, t, tcluster)
+						setupExternalTest(h, t, newTC)
 						// Collect the journal logs after execution is finished
-						defer collectLogsExternalTest(h, t, tcluster)
+						defer collectLogsExternalTest(h, t, newTC)
 					}
 
-					newTC := cluster.TestCluster{
-						H:       h,
-						Cluster: tcluster.Cluster,
-					}
 					t.Run(newTC)
 				}
 				// Each non-exclusive test is run as a subtest of this wrapper test