test(concat_string): add tests for exercise

sh/tests/concat_string_test.py

@@ -0,0 +1,13 @@
+import sys
+
+sys.path.append('student')
+
+from concat_string import concat
+
+def test_concat():
+    assert concat("Hello", "World") == "Hello, World"
+    assert concat("Hi", "there") == "Hi, there"
+    assert concat("", "") == ", "
+    assert concat("Hello", "") == "Hello, "
+    assert concat("", "World") == ", World"
+test_concat()