Improve Copilot instructions and add test guidelines #1191

Job	Run time
build	1m 6s
test-unit (18, macos)	1m 37s
lint	1m 7s
test-integration (18, ubuntu-22.04, chromium)	3m 40s
test-unit (18, windows)	7m 57s
test-unit (18, ubuntu)	1m 39s
test-integration (18, ubuntu-22.04, firefox)	3m 45s
test-integration (18, ubuntu-22.04, webkit)	3m 56s
release-dry-run (18)	2m 21s
test-unit-coverage	1m 17s
	28m 25s

Provide feedback