[client] fix privileged test CI failures and run the harness on macOS

The host-safe unit run dropped sudo but two privileged test groups were
never tagged, and the Docker privileged job silently never ran the suite:

- Gate the ssh/server PrivilegeDropper command-construction tests behind
  the privileged tag (they require root to target a different UID); split
  them into executor_unix_privileged_test.go.
- Tag sharedsock raw-socket tests privileged (need CAP_NET_RAW).
- Fix the Docker job command: nested single quotes around the build tags
  closed the sh -c wrapper early, dropping the go list package set and the
  privileged tag, so go test ran on the empty repo root. Use double quotes.

Make the self-hosting harness usable from a dev Mac:

- Build it on darwin as well as linux; it only drives Docker.
- Resolve the active docker context endpoint into DOCKER_HOST when the
  default /var/run/docker.sock is absent (Docker Desktop, Colima, OrbStack).
- Rename the misspelled containerGoModache constant to containerGoModCache.

.github/workflows/golang-test-linux.yml

@@ -229,7 +229,7 @@ jobs:
             sh -c ' \
               apk update; apk add --no-cache \
                 ca-certificates iptables ip6tables dbus dbus-dev libpcap-dev build-base; \
-              go test -buildvcs=false -tags 'devcert privileged' -v -timeout 10m -p 1 $(go list -buildvcs=false ./... | grep -v -e /management -e /signal -e /relay -e /proxy -e /combined -e /client/ui -e /upload-server -e /client/testutil/privileged)
+              go test -buildvcs=false -tags "devcert privileged" -v -timeout 10m -p 1 $(go list -buildvcs=false ./... | grep -v -e /management -e /signal -e /relay -e /proxy -e /combined -e /client/ui -e /upload-server -e /client/testutil/privileged)
             '
 
   test_relay: