unit tests for lognormal distribution (#71)

Addressing #1

tests/lognormal_test.py

@@ -0,0 +1,27 @@
+#  Copyright 2023 The Jaxampler Authors
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+
+import sys
+
+import pytest
+
+sys.path.append("../jaxampler")
+from jaxampler.rvs import LogNormal
+
+
+class TestLogNormal:
+    def test_invalid_params(self):
+        with pytest.raises(AssertionError):
+            LogNormal(sigma=-1.0)