Add additional test for mdadm_linux collector

collector/fixtures/proc/mdstat_invalid

@@ -0,0 +1,5 @@
+Personalities : [invalid]
+md3 : invalid
+      314159265 blocks 64k chunks
+
+unused devices: <none>

collector/mdadm_linux_test.go

@@ -46,3 +46,11 @@ func TestMdadm(t *testing.T) {
 		t.Errorf("expected number of parsed md-device to be %d, but was %d", len(refs), len(mdStates))
 	}
 }
+
+func TestInvalidMdstat(t *testing.T) {
+	_, err := parseMdstat("fixtures/proc/mdstat_invalid")
+
+	if err == nil {
+		t.Fatalf("parsing of invalid reference file did not find any errors")
+	}
+}