spread test skips tests with same name prefixes

[zhijie-yang]

In the spread.yaml workflow line 65, a regex is used to prevent duplicated tests being passed to the spread command.

However, this regex gives false negatives in certain scenarios:

Suppose the changed files include slices/golang-1.22-go.yaml and tests/spread/integration/golang/task.yaml.

The variable $spread_tasks will be filled with lxd:tests/spread/integration/golang-1.22-go. When tests/spread/integration/golang is taken in to the expression $spread_tasks != *"${pkg_tests}"*, it evaluates to false, since lxd:tests/spread/integration/golang-1.22-go equals to the pattern *tests/spread/integration/golang*.

In this case, although changes are made to tests/spread/integration/golang/task.yaml, the tests won't never run, since lxd:tests/spread/integration/golang-1.22-go takes the precedence.

A workflow run can also prove this behaviour.

[lczyk]

adding priority since its a serious bug which caused us to commit broken slices. fixed v quickly in #815 w/out regressions