dev-libs/wayland-9999: Port to EAPI-6

dev-libs/wayland-9999: Port to EAPI-6

Also try to address socket path 108 byte limit restriction caused test
failures by cutting down the XDG_RUNTIME_DIR subdir length.
Wayland tests used to create a wayland-tests/ subdir under it, but since
before 1.13.0 it creates a wayland-tests-RANDOM/ subdir (RANDOM is 6
random characters), which makes tests fail on socket path length even with
PORTAGE_TMPDIR=/tmp, let alone /var/tmp.
I think shortening the subdir from "runtime-dir" to "xdr" makes it fit
in both cases, unless we are dealing with a 1.xx.y-r1 revision, in which
case /var/tmp might fail.