tstest/integration: set race flag when cross compiling, conditionally fail on race

Misc cleanups and things noticed while working on #7894 and pulled out
of a separate change. Submitting them on their own to not distract
from later changes.

Updates #7894

Change-Id: Ie9abc8b88f121c559aeeb7e74db2aa532eb84d3d
Co-authored-by: Maisem Ali <maisem@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
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