tests: duplicate test for pager for old extension and for in-core pager When the old pager extension is enabled, I think we should try to be as BC as reasonable. To help with that, this patch brings back test-pager.t as of 65a3b4d67a65 (pager: add a test of --pager=no functionality, 2017-02-06), but under the name test-pager-legacy.t However, since the behavior has changed in a few cases (notably by no longer respecting pager.attend), the file is modified to work with the current version. We will recover some lost BC in coming patches. Also, to make sure the in-core pager does not depend on the pager extension being enabled, this patch disables the extension in test-pager.t. It turns out that pager.attend-$cmd was only supported when the pager extension was enabled, so the tests are updated to reflect that. We will need to decide what to do with these.