test-claw-master-server.py: two tests read the caller's claude-master environment #678

On main, python3 -S -B scripts/test-claw-master-server.py fails two tests when run on a dev box where the claude-master client is configured:

• test_nextjs_dev_installs_the_pinned_client_and_routes_only_enrolled_accounts
• test_the_metal_box_gets_the_pinned_client_and_routing_through_an_ssm_association

Each expects 'unset|unset' and instead gets the box's real server address and client path, so the scripts under test inherit the caller's environment (the claude-master base URL and client variables) rather than a cleared one. On a host without those variables, both pass.

Fix: run the script under test with an explicit environment (only PATH, HOME and what the test sets), or unset the claude-master variables in the test's subprocess env, so the result does not depend on where the suite runs. Found while testing #675; the failures are identical on main without that change.