test: apply GO_TEST_TIMEOUT_SCALE scaling to test timeouts

Change run.go to apply the GO_TEST_TIMEOUT_SCALE scaling factor to
test timeouts (mentioned in "-t" clause in test header).

Also with this patch, bump up the timeout for fixedbugs/issue46234.go
from 30 to 45 seconds, to avoid flakes on very slow builders.

Updates #50973.

Change-Id: Icbafa482860e24cc1e72fee53511bcc764d06bf1
Reviewed-on: https://go-review.googlesource.com/c/go/+/382774


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>