Add unit tests for mypy plugin

* Add test cases for issues jspahrsummers#21, jspahrsummers#25, jspahrsummers#26
* Test case for jspahrsummers#21 is active because it passes with mypy==0.711
* Test cases for jspahrsummers#25 and jspahrsummers#26 are deactivated because they fail

tests/source_files/issue21.py

@@ -0,0 +1,9 @@
+from adt import adt, Case
+
+
+@adt
+class Cmd:
+    CASE0: Case
+    CASE1: Case[str]
+    CASE2: Case[str, str]
+    CASE3: Case[str, str, str]

tests/source_files/issue25.py

@@ -0,0 +1,9 @@
+from adt import adt, Case
+
+
+@adt
+class Expression:
+    LITERAL: Case[float]
+
+
+result: None = Expression.LITERAL(0.1).match(literal=lambda n: None)

tests/source_files/issue26.py

@@ -0,0 +1,15 @@
+from typing import Optional
+from adt import adt, Case
+
+
+@adt
+class ExampleADT:
+    EMPTY: Case
+    INTEGER: Case[int]
+    STRING_PAIR: Case[str, str]
+
+    @property
+    def safe_integer(self) -> Optional[int]:
+        return self.match(empty=lambda: None,
+                          integer=lambda n: n,
+                          string_pair=lambda a, b: None)

tests/test_mypy_plugin.py

@@ -0,0 +1,60 @@
+import unittest
+import mypy.main
+import mypy.version
+import os
+import sys
+import io
+
+
+class SimpleBuffer(io.TextIOBase):
+    """Simple memory buffer that behaves like a file"""
+
+    def __init__(self):
+        self.__buffer = []
+
+    def write(self, text):
+        self.__buffer.append(text)
+        print(text, end="")
+        return len(text)
+
+    def read(self):
+        return ''.join(self.__buffer)
+
+    def __str__(self):
+        return self.read()
+
+
+class TestMyPyPlugin(unittest.TestCase):
+    def test_issue21(self) -> None:
+        self._call_mypy_on_source_file("issue21.py")
+
+    def _test_issue25(self) -> None:
+        # TODO: This is a deactivated test for issue #25.
+        # Activate it when working on it.
+        # cf. https://github.com/jspahrsummers/adt/issues/25
+        self._call_mypy_on_source_file("issue25.py")
+
+    def _test_issue26(self) -> None:
+        # TODO: This is a deactivated test for issue #26.
+        # Activate it when working on it.
+        # cf. https://github.com/jspahrsummers/adt/issues/26
+        self._call_mypy_on_source_file("issue26.py")
+
+    def _test_readme_examples(self) -> None:
+        # TODO: This fails because of issue #26. Deactivated this test.
+        self._call_mypy_on_source_file("readme_examples.py")
+
+    def _call_mypy_on_source_file(self, source_file_name: str):
+        print(
+            f"Testing {source_file_name} with mypy=={mypy.version.__version__}"
+        )
+        self._call_mypy_on_path(
+            os.path.join(os.path.dirname(__file__), "source_files",
+                         source_file_name))
+
+    def _call_mypy_on_path(self, testfile):
+        output = SimpleBuffer()
+        try:
+            mypy.main.main(None, output, sys.stderr, args=[testfile])
+        except SystemExit:
+            self.fail(msg="Error during type-check:\n{}".format(output))