The existing code relied on the Go build cache to avoid
needless work when obtaining the tailscale binaries.
For non-obvious reasons, the binaries were getting re-linked
every time, which added 600ms or so on my machine to every test.
Instead, build the binaries exactly once, on demand.
This reduces the time to run 'go test -count=5' from 34s to 10s
on my machine.
Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
Josh Bleecher Snyder
4691e012a9