test_util: Add meaningful names to test LogContexts

Currently, when a test creates a new LogContext, a numeric counter is incremented every time a test creates a new LogContext, and the resultant path will be of the form {test_name}.{pid}.{id}.log. The counter is useful as it ensures that multiple LogContexts can coexist within a single test execution. However, it doesn't really help with identifying what entity in the test created that log file. Instead, this must be inferred by reading the test code to determine the order in which LogContexts are created, which can sometimes be challenging --- especially when there's a bunch of test support code spinning up multiple Dropshot servers.

It would be nice if there was a way to add a human-readable test component identifier to these log paths, like "server" or "client". The counter could still be used in addition to such an additional identifier to ensure all paths are unique.

Start in the test_util code that creates LogContext paths and inspect the test sites that create multiple contexts, especially support code for Dropshot servers. Define how a human-readable component identifier is supplied and verify that generated paths include it while the numeric counter still keeps paths unique.