Support for parallel tests

[ansel1]

Improves support for parallel tests in v2:

• feat: flume v2 adds a new middleware and helper function for adding log attributes via the context. The helper attaches log attributes to a context object, and the middleware extract those attributes and adds them to log entries. For example: attach a request ID to the context object, and that request ID will be added to every log entry call when that context is passed to the log method.
• fix: flumetest v2 didn't work with parallel tests: each test would start capturing the global log stream. Parallel tests would keep grabbing the log stream away from each other. Now, each test subscribes to the global log stream, allowing parallel tests to subscribe simultaneously. This does mean that individual log lines may appear in the log captures for multiple tests, and for any individual test, it will capture log lines which may not be related to that test's activities, but at least each test can guarantee that it has captured a full superset of the logs related to that test.
• feat: flumetest v2 adds a new "Capture()" method, which is meant to supercede Start(). The only differences are:
  • It does not return a revert function. It exclusively uses t.Cleanup() to flush the log capture buffer. The does mean that in certain panic situations, the logs may be lost, but that is expected to be fixed in the golang SDK soon, and this makes Capture() much easier to embed in other test setup helper methods than Start().
  • It automatically installs the ContextAttrsMiddleware in the global slog handler, and returns a context.Context object (derived from t.Context()) which embeds the name of the test in it.