Fix test_rendering flaky pager test

[tompng]

Fixes this random failing test: https://github.com/ruby/irb/actions/runs/20289200418/job/58269730542

Looks like IRB evaluates this two lines at the same time.

'a' * 80 * 11
'foo' + 'bar'

We need to first make IRB evaluate 'a' * 80 * 11, wait for pager to launch (with assert_screen(/a{80}/)), and then input the second line.