cmd/gobind: exec the test binary as gobind and log only stderr by default
cmd/gobind: exec the test binary as gobind and log only stderr by default

Using the test binary as the "gobind" command not only avoids the
overhead of recompiling the command, but also allows commands like "go
test -race" to actually test the requested configuration of the
command.

Logging stderr and stdout separately — and logging only stderr by
default — makes the failure messages much easier to spot. (Logging the
combined output as before produced a massive wall of text that tends
to bury the actual errors.)

For golang/go#56292.

Change-Id: Ia11fad19418d9b9004608c76fe512ceab4f247bc
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/443655
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Hajime Hoshi <1c55649263e1aefbe86bfa40d3d8351033a4d83f@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Auto-Submit: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

golang/mobile,golang/mobile,golang/mobile,golang/mobile