runtime: replace --buildmode with -buildmode in tests

While debugging issue #45638, I discovered that some tests were using
--buildmode command line parameter instead of -buildmode.

The --buildmode parameter is handled properly by the flag package - it
is read as -buildmode. But we should correct code anyway.

Updates #45638

Change-Id: I75cf95c7d11dcdf4aeccf568b2dea77bd8942352
Reviewed-on: https://go-review.googlesource.com/c/go/+/313351


Trust: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>