runtime: eliminate arbitrary timeouts in runBuiltTestProg and TestGdbBacktrace

runtime: eliminate arbitrary timeouts in runBuiltTestProg and TestGdbBacktrace

This may fix the TestEINTR failures that have been frequent on the
riscv64 builders since CL 445597.

Updates #37405.
Updates #39043.

Change-Id: Iaf1403ff5ce2ff0203d5d0059908097d32d0b217
Reviewed-on: https://go-review.googlesource.com/c/go/+/447495
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>