Add installation tests and functionality tests

Some test coverage extracted from 🐛 Cannot edit Payflow / Payflow Link payment gateways after creation, requires password be entered each time Needs review .

Improved coverage.

This fixes a bug with the config schema - there wasn't an entry for "cancel_link".

When I run #4 locally, after the edit form is submitted again it doesn't return to the payments list, it just shows the edit form again.

The last submitted patch, 4: commerce_paypal-n3322061-4.patch, failed testing. View results →
- codesniffer_fixes.patch Interdiff of automated coding standards fixes only.

It seems like 🐛 Cannot edit Payflow / Payflow Link payment gateways after creation, requires password be entered each time Needs review resolves the problem that makes the tests pass.

FYI the cancel_link issue comes from when it was originally added in #3185624: Add on-site iframe support to Payflow Link → .