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Previously when starting subprocesses test launcher would clone
some namespace dirs, but not all of them. Update test launcher to use
fdio_ns_export_root() to get namespace of the current process so the
child process can get complete namespace clone, except for /data that
needs to be overridden.

Change-Id: I0e49aa12a4a98e036acdf6549252478e6c771936
Reviewed-on: https://chromium-review.googlesource.com/c/1351888
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611815}