Do not use the OSCrypt mocker for Linux browser tests

PrepareBrowserCommandLineForTests appends the --password-store=basic
command line switch so the mocker is not needed, since the basic
password store never blocks.

This CL improves test coverage, as it means the OSCrypt initialization
can occur as normal.

Bug: 389016528

Change-Id: Icf08b46badd94f4401663cb4c44825a24bcbf647
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6244107
Commit-Queue: Will Harris <wfh@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1419609}