test(async): replace sleeps with event-driven waits

Several functional specs were carrying duplicated helpers, arbitrary
sleep calls and retry wrappers to paper over asynchronous gitsigns
updates. That made the tests slower and still left them vulnerable to
timing-dependent failures.

Move the shared attach, highlight, and command wait helpers into
test/gs_helpers.lua and update the touched specs to use them instead
of local copies. Command-driven tests now wait for the next
GitSignsUpdate event, attach-heavy tests wait for cache and status
state to settle, and the repo lock test now uses async events instead
of timer sleeps.