net: remove an arbitrary timeout in TestUDPReadSizeError

Looking at the condition actually exercised by the test it seems
unnecessary: assuming that the Write succeeds (checked earlier in the
test), the Read must have a nonzero number of bytes available to read
immediately. (That is not the case in TestUDPZeroByteBuffer, from
which this test appears to have been derived.)

Fixes #50870

Change-Id: Ia6040a2d5dc320f0b86ec9d6f6b91dc72e8f3b84
Reviewed-on: https://go-review.googlesource.com/c/go/+/382537


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>