Make assertSnapshot compatible with execution outside the main thread

[sk409]

assertSnapshot uses XCTContext.runActivity to save attachments, but XCTContext.runActivity crashes when run outside of the main thread.
Also, assertSnapshot blocks the thread if I use XCTWaiter.wait in Snapshotting, but I use a library that does processing in the main thread, so assertSnapshot(XCTWaiter.wait) must be executed outside the main thread so that XCTWaiter.wait doesn't block Library.

Version:

swift-snapshot-testing: 1.17.4