Keyboard input in the test suite is flaky on Wayland

The following discussion from !838 (merged) should be addressed:

• @intrigeri started a discussion: (+4 comments)

  • test suite does not find the DuckDuckGo search prompt, while it's visible on the failure screenshot

The code that's involved is:

• our own Screen#press calls https://libvirt.org/ruby/api/Libvirt/Domain.html#method-i-send_key
• https://libvirt.org/ruby/api/Libvirt/Domain.html#method-i-send_key calls https://www.libvirt.org/html/libvirt-libvirt-domain.html#virDomainSendKey
• https://www.libvirt.org/html/libvirt-libvirt-domain.html#virDomainSendKey calls conn->driver->domainSendKey, which in our case is qemuDomainSendKey (https://sources.debian.org/src/libvirt/8.10.0-1/src/qemu/qemu_driver.c/?hl=20813#L20813)
• qemuDomainSendKey → qemuMonitorSendKey → qemuMonitorJSONSendKey, which calls qemuMonitorJSONMakeCommand with the send-key argument, and passes its result to qemuMonitorJSONCommand; all this interacts with the "QEMU monitor console"
• eventually we end up in hmp_sendkey → qmp_sendkey → qemu_input_event_send_key_delay, which uses kbd_default_delay_ms = 10 as the default hold time

Note: the doc says the default hold time is 100 ms, which was indeed the case initially, but since https://salsa.debian.org/qemu-team/qemu/-/commit/2e377f1730d06deafb3e3ef6cf88792de4a6f4df it uses kbd_default_delay_ms, which has been 10 ms since it was introduced in 2014.