parse: apkindex: fix provider_priority never actually being an int

provider_priority was actually a string all along! make it an int as type hinting suggests, and add some tests for apkindex parsing.

cc: @Newbyte

pmb/chroot/test_apk.py

@@ -21,7 +21,7 @@ def apk_mocks(monkeypatch) -> dict | None:
 
     def _apkindex_package(
         _package: str, _arch: Arch, _must_exist: bool = False, indexes=None
-    ) -> ApkindexBlock:
+    ) -> ApkindexBlock | None:
         if _package == "package1":
             return ApkindexBlock(
                 arch=_arch,
@@ -68,6 +68,8 @@ def apk_mocks(monkeypatch) -> dict | None:
                 version="5.5-r0",
             )
 
+        return None
+
     monkeypatch.setattr(pmb.parse.apkindex, "package", _apkindex_package)
     return None