add basic test of overall plaited functionality

- runs a actual piece of code through the plaited filter and
  checks its output

tests/test_plaited.py

@@ -1,4 +1,63 @@
+import io
+import sys
+import json
+from contextlib import redirect_stdout
+
 import pytest
 import panflute as pf
+from plaited.plaited import main
+
+
+@pytest.fixture
+def basic_code_test():
+    test_doc = '#a\nb\n```python\nprint("c")\n```\nd'
+
+    expected_input = pf.convert_text(test_doc, output_format="json").encode("utf-8")
+
+    # leave out the pandoc-api-version key, because it will vary by environment
+    expected_output = {
+        "meta": {"tables": {"t": "MetaBool", "c": True}},
+        "blocks": [
+            {
+                "t": "Para",
+                "c": [
+                    {"t": "Str", "c": "#a"},
+                    {"t": "SoftBreak"},
+                    {"t": "Str", "c": "b"},
+                ],
+            },
+            {"t": "CodeBlock", "c": [["", ["python"], []], 'print("c")']},
+            {"t": "LineBlock", "c": [[{"t": "Str", "c": "c"}]]},
+            {"t": "Para", "c": [{"t": "Str", "c": "d"}]},
+        ],
+    }
+
+    return {"input": expected_input, "output": expected_output}
+
+
+def test_main(basic_code_test):
+    # this will pretend to be sys.stdin.buffer
+    dummy_reader = io.BufferedReader(io.BytesIO(basic_code_test["input"]))
+
+    # sys.stdin.buffer is a read-only attribute, but I want to overwrite it
+    # so, here is a dummy class to replace sys.stdin entirely
+    # what could go wrong?
+    class DummyStdin:
+        buffer = dummy_reader
+
+    # no-one will notice...
+    sys.stdin = DummyStdin()
+
+    # ...heh but that's not all
+    # I want to also trick panflute into writing out into this test
+    dummy_stdout_buffer = io.BytesIO()
+    with redirect_stdout(io.TextIOWrapper(dummy_stdout_buffer)) as f:
+        main()
+
+    # parse the output as json, and create a dictionary
+    json_output = json.loads(dummy_stdout_buffer.getvalue().decode("utf-8"))
+
+    # remove pandoc-api-version, this varies by environment
+    del json_output["pandoc-api-version"]
 
-# call main() in subprocess, pipe inputs into it, and see if we get correct output
+    assert json_output == basic_code_test["output"]