[gha] remove stdout from test logs to reduce verbosity

[andrurogerz]

Purpose

Reduce the unnecessary logging produced when running lldb-dotest.

Overview

• Suppress stdout when running lldb-dotest since all of the useful output, including pass/fail logs, goes to stderr
• Reduces the log size so that they entirely fit in the GitHub log view page-- there is no need to download the log archive to find which test case failed
• Reduces the test run time by multiple minutes

Validation

Run GitHub build & test job on this PR.