Move AllowCheckIsTestForTesting to be before NetworkServiceTestHelper

In some test libraries (browser_tests, chromeos browser_tests, and
headless_browsertests, along with anything that inherits from them), the
NetworkServiceTestHelper is created prior to the call to
content::LaunchTests(). NetworkServiceTestHelper is test-specific code
but currently cannot reference anything that uses CHECK_IS_TEST()
because AllowCheckIsTestForTesting() is not called until LaunchTests().

This change adds calls to AllowCheckIsTestForTesting() in these three
locations. It also removes the reentrancy CHECK in
AllowCheckIsTestForTesting() as it isn't really necessary. There are
linter checks to prevent production code from reference *ForTesting
methods.

Bug: 404286909
Change-Id: I4dd90d0a21e034c6df99c2ab16fe3cf9a63d7b2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6363478
Reviewed-by: Victor Vianna <victorvianna@google.com>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Cliff Smolinsky <cliffsmo@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1436970}