Add configurable library reference roots

tests/test_config.py

@@ -0,0 +1,17 @@
+from pathlib import Path
+
+from serato_doctor.config import ScanConfig
+
+
+def test_scan_config_freezes_reference_roots():
+    roots = (Path(root) for root in ["/old/one", "/old/two"])
+
+    config = ScanConfig.build(
+        serato=Path("serato"),
+        music=Path("music"),
+        out=Path("scan.csv"),
+        report=Path("report.txt"),
+        reference_roots=roots,
+    )
+
+    assert config.reference_roots == (Path("/old/one"), Path("/old/two"))