test: make the 'reference-image' file optional
test: make the 'reference-image' file optional

'reference-image' is used only for pixel tests, and currently also only
in some cases (e.g. only for chrom* browsers and non-devel builds).

Simplify its usage a bit: read it only when needed, and consider its
lack non-fatal (situation handled already).

cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit