Is your feature request related to a problem? Please describe.

fail-rs utilizes global registry to expose simple APIs and convenient FailPoint definition. But it also means all parallel tests have to be run one by one and do cleanup between each run to avoid configurations affect each other.

Describe the solution you'd like

This issue proposes to utilize thread group. Each test case defines a unique thread group, all configuration will be bound to exact one thread group. Every time a new thread is spawn, it needs to be registered to one thread group to make FailPoint reads configurations. If a thread is not registered to any group, it belongs to a default global group.

New public APIs include:

pub fn current_fail_group() -> FailGroup;

impl FailGroup {
    pub fn register_current(&self) -> Result<()>;
    pub fn deregister_current(&self);
}

Note that it doesn't require users have the ability to spawn a thread, register the thread before using FailPoint is enough.

Describe alternatives you've considered

One solution to this is pass the global registry to struct constructor, but it will interfere the general code heavily, it needs to be passed to anywhere FailPoints are defined.

Another solution is #24, but it lacks threaded cases support.