How can I pass a ctx (Context) to CliRunner?

You would directly pass your Config instance as keyword argument obj to runner.invoke:

import click
from click.testing import CliRunner

class Config():
    def __init__(self):
        self.value = 651

@click.command()
@click.pass_obj
def print_numberinfo(obj):
    if not hasattr(obj, 'value'):
        obj = Config()
    click.echo(obj.value)

def test_print_numberinfo():
    obj = Config()
    obj.value = 777
    runner = CliRunner()
    # how do I pass ctx to runner.invoke?
    result = runner.invoke(print_numberinfo, obj=obj)
    assert result.output == str(obj.value) + '\n'