Add regression test for postprocess hook

Test demonstrates a simple real world multitoken transformation
from: asyncio.current_task() to: current_thread()
it also has data to confirm that those tokens on their own are not touched
(so asyncio remains asyncio and curent_task() remains current_task())

Author: Tim Hedger-Gourlay

tests/data/postprocess/async/hello.py

@@ -0,0 +1,3 @@
+self._update_task = asyncio.current_task()
+asyncio
+current_task()

tests/data/postprocess/sync/hello.py

@@ -0,0 +1,3 @@
+self._update_task = current_thread()
+asyncio
+current_task()

tests/test_post_process.py

@@ -0,0 +1,66 @@
+import os
+
+from token import NAME
+from tokenize import TokenInfo
+
+from unasync import Rule
+
+TEST_DIR = os.path.join(os.path.dirname(os.path.abspath(__file__)), "data")
+TEST_DIR = os.path.join(TEST_DIR, "postprocess")
+ASYNC_DIR = os.path.join(TEST_DIR, "async")
+SYNC_DIR = os.path.join(TEST_DIR, "sync")
+TEST_FILES = sorted(f for f in os.listdir(ASYNC_DIR) if f.endswith(".py"))
+
+class PostProcessRule(Rule):
+
+    def _postprocess_tokens(self, tokens):
+        # Replace:
+        #     asyncio.current_task()
+        # with:
+        #     current_thread()
+
+        prev2 = None
+        prev1 = None
+
+        for token in tokens:
+
+            if (
+                prev2 is not None
+                and prev2.src == "asyncio"
+                and prev1.src == "."
+                and token.src == "current_task"
+            ):
+                yield token._replace(src="current_thread")
+
+                prev2 = None
+                prev1 = None
+
+            elif prev2 is not None:
+                yield prev2
+                prev2 = prev1
+                prev1 = token
+
+            else:
+                prev2 = prev1
+                prev1 = token
+
+        if prev2 is not None:
+            yield prev2
+        if prev1 is not None:
+            yield prev1
+
+
+def test_postprocess(tmpdir):
+    rule = PostProcessRule(fromdir=ASYNC_DIR, todir=str(tmpdir))
+
+    for source_file in TEST_FILES:
+        rule._unasync_file(os.path.join(ASYNC_DIR, source_file))
+
+    for source_file in TEST_FILES:
+        with open(os.path.join(SYNC_DIR, source_file)) as f:
+            truth = f.read()
+
+        with open(os.path.join(str(tmpdir), source_file)) as f:
+            unasynced = f.read()
+
+        assert unasynced == truth