selfupdate: fix "invalid hashsum signature" error

This was caused by a change to the upstream library
ProtonMail/go-crypto checking the flags on the keys more strictly.

However the signing key for rclone is very old and does not have those
flags. Adding those flags using `gpg --edit-key` and then the
`change-usage` subcommand to remove, save, quite then re-add, save
quit the signing capabilities caused the key to work.

This also adds tests for the verification and adds the selfupdate
tests into the integration test harness as they had been disabled on
CI because they rely on external sources and are sometimes unreliable.

Fixes #7373

cmd/selfupdate/testdata/verify/SHA256SUMS

@@ -0,0 +1,10 @@
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA1
+
+b20b47f579a2c790ca752fb5d8e5651fade7d5867cbac0a4f71e805fc5c468d0  archive.zip
+-----BEGIN PGP SIGNATURE-----
+
+iF0EARECAB0WIQT79zfs6firGGBL0qyTk14C/ztU+gUCZS+oVQAKCRCTk14C/ztU
++lNsAJ9XRiODlM4fIW9yqiltO3N+lLeucwCfRzD3cXk6BCB5wdz7pTgnItk9N74=
+=1GTr
+-----END PGP SIGNATURE-----