Better testing of PerfData.

I wouldn't call it perfect or thorough, but it's a little better than
before.

tests/test_nagios.py

@@ -113,4 +113,13 @@ def test_output_parsing(output):
 @pytest.mark.parametrize("line", PERF_DATA)
 def test_data_perf(line):
     """Test parsing of perfdata."""
-    nagios.PerfData(line)
+    pd = nagios.PerfData(line)
+    assert isinstance(pd.name, str)
+    assert len(pd.name) > 0
+    assert isinstance(pd.value, float)
+    assert isinstance(pd.unit, str)
+    assert len(pd.unit) > 0
+    assert isinstance(pd.warning, (float, type(None)))
+    assert isinstance(pd.critical, (float, type(None)))
+    assert isinstance(pd.min, (float, type(None)))
+    assert isinstance(pd.max, (float, type(None)))