tests: add decompress() tests for multiple frames and extra data

Related to #59 and #181. The existing behavior isn't great. But we
should at least have tests to keep us honest if/when behavior changes.

tests/test_decompressor_decompress.py

@@ -175,3 +175,18 @@ def test_explicit_default_params(self):
             format=zstd.FORMAT_ZSTD1,
         )
         self.assertEqual(dctx.decompress(compressed), b"foo")
+
+    def test_multiple_frames(self):
+        cctx = zstd.ZstdCompressor()
+        foo = cctx.compress(b"foo")
+        bar = cctx.compress(b"bar")
+
+        dctx = zstd.ZstdDecompressor()
+        self.assertEqual(dctx.decompress(foo + bar), b"foo")
+
+    def test_junk_after_frame(self):
+        cctx = zstd.ZstdCompressor()
+        frame = cctx.compress(b"foo")
+
+        dctx = zstd.ZstdDecompressor()
+        self.assertEqual(dctx.decompress(frame + b"junk"), b"foo")