jsonmessage: added test and cleaned up others

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)

utils/jsonmessage_test.go

@@ -17,13 +17,22 @@ func TestProgress(t *testing.T) {
 		t.Fatalf("Expected empty string, got '%s'", jp.String())
 	}
 
+	expected := "     1 B"
 	jp2 := JSONProgress{Current: 1}
-	if jp2.String() != "     1 B" {
-		t.Fatalf("Expected '     1 B', got '%s'", jp2.String())
+	if jp2.String() != expected {
+		t.Fatalf("Expected %q, got %q", expected, jp2.String())
 	}
 
+	expected = "[=========================>                         ]     50 B/100 B"
 	jp3 := JSONProgress{Current: 50, Total: 100}
-	if jp3.String() != "[=========================>                         ]     50 B/100 B" {
-		t.Fatalf("Expected '[=========================>                         ]     50 B/100 B', got '%s'", jp3.String())
+	if jp3.String() != expected {
+		t.Fatalf("Expected %q, got %q", expected, jp3.String())
+	}
+
+	// this number can't be negetive gh#7136
+	expected = "[==============================================================>]     50 B/40 B"
+	jp4 := JSONProgress{Current: 50, Total: 40}
+	if jp4.String() != expected {
+		t.Fatalf("Expected %q, got %q", expected, jp4.String())
 	}
 }