Migrate LogTest to LoggingConfig

InitializeLogging() succeeds only once per process, so the logging
initialization tests ran as EXPECT_EXIT death tests and relied on a
WEBRTC_TEST_SKIP_LOGGING_INIT environment variable in
TestMainImpl::Init(). The other LogTest tests used legacy static setters
such as AddLogToStream(), SetLogQueueNames() and the deprecated
LogThreads().

- Add ResetLoggingForTesting(). It swaps the active LoggingConfig for a
  default one and lets the next InitializeLogging() call succeed, or
  restores a previously saved config. The test-only state is only read
  by InitializeLogging(), so the per-message logging path is unchanged.
- Convert LogTest to a fixture that saves and restores the config set up
  by TestMainImpl::Init() and logs through LoggingConfig sinks. Remove
  the death tests, the environment variable, a duplicate queue name test
  and ImplicitInitialization, which cannot run in-process and whose
  death-test version passed regardless.
- Filter LoggingConfig sinks by LoggingConfig::min_severity(). They were
  filtered by LogSink::min_severity_, which only AddLogToStream() sets,
  so config sinks always used LS_INFO and kept the global threshold at
  LS_INFO or lower.
- Log the --verbose warning in TestMainImpl::Init() after
  InitializeLogging(). Logging it first triggered implicit
  initialization in debug builds, so the logging flags were ignored.

Bug: webrtc:42234107
Change-Id: Icd992a3e1d0e252df09ccf52609ff91106eecd97
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/510041
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#49043}