Make EmbeddedTestServer logging more aggressive

This CL:

 - Changes every instance of DCHECK in EmbeddedTestServer to CHECK;
 - Changes every instance of DVLOG to VLOG;
 - Upgrades some initialization-related failure VLOGs (which are especially
   important and are not expected to be spammy) to LOG(ERROR).

The rationale is:

 - The use of DCHECK and DVLOG doesn't seem to make sense for test code,
   as test code is not expected to be performance critical;
 - Hiding test server initialization failures behind VLOG(1) makes it
   extra hard to figure out why the test server is not working, as
   evidenced in https://crbug.com/433957945.

Bug: 433957945
Change-Id: I9d6588e035c8f3f62aec5a658c6713615e2385d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6797152
Auto-Submit: Etienne Dechamps <edechamps@google.com>
Commit-Queue: mmenke <mmenke@chromium.org>
Reviewed-by: mmenke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1493594}