Also fix a (minor) double-Close error in Serve that was exposed by the
test fix.

Serve accepts a net.Listener, which produces net.Conn instances.
The documentation for net.Conn requires its methods to be safe for
concurrent use, so most implementations likely allow Close to be
called multiple times as a side effect of making it safe to call
concurrently with other methods. However, the net.Conn interface is a
superset of the io.Closer interface, io.Closer explicitly leaves the
behavior of multiple Close calls undefined, and net.Conn does not
explicitly document a stricter requirement.

Perhaps more importantly, the test for the fcgi package calls
unexported functions that accept an io.ReadWriteCloser (not a
net.Conn), and at least one of the test-helper ReadWriteCloser
implementations expects Close to be called only once.

The goroutine leaks were exposed by a racy arbitrary timeout reported
in golang#61271. Fixing the goroutine leak exposed the double-Close error:
one of the leaked goroutines was blocked on reading from an unclosed
pipe. Closing the pipe (to unblock the goroutine) triggered the second
Close call.

Fixes golang#61271.

Change-Id: I5cfac8870e4bb4f13adeee48910d165dbd4b76fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/508815
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>