Override requirements (at a given level) from a requirements file. In…

… that case, instead of merging, we override

src/briefcase/config.py

@@ -1,5 +1,6 @@
 import copy
 import keyword
+import pathlib
 import re
 import unicodedata
 from types import SimpleNamespace
@@ -487,6 +488,14 @@ def merge_config(config, data):
 
         if value:
             config.setdefault(option, []).extend(value)
+    for prefix in ["", "test_"]:
+        try:
+            fname = data.pop(f"{prefix}requires_file")
+        except KeyError:
+            pass
+        else:
+            value = pathlib.Path(fname).read_text().splitlines()
+            config[f"{prefix}requires"] = value
 
     config.update(data)