t5516/t5601: be less strict about the number of credential warnings

t5516/t5601: be less strict about the number of credential warnings

It is unclear as to _why_, but under certain circumstances the warning
about credentials being passed as part of the URL seems to be swallowed
by the `git remote-https` helper in the Windows jobs of Git's CI builds.

Since it is not actually important how many times Git prints the
warning/error message, as long as it prints it at least once, let's just
make the test a bit more lenient and test for the latter instead of the
former, which works around these CI issues.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Jeff King <redacted>
Signed-off-by: Taylor Blau <redacted>