testplan: Parse builds from a file instead of CLI.

- Calls to get-testable with too many builds were causing OS
errors because the arguments were too long. Add an option to
parse builds from a file instead of on the command line.

TEST=Manually ran `testplan get-testable`

Change-Id: I01dea1f23de32e105d283c0d9a6dcf72c53bfff7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/4568625
Reviewed-by: Chris Gerber <gerb@google.com>
Commit-Queue: Chris Gerber <gerb@google.com>
Auto-Submit: Andrew Lamb <andrewlamb@chromium.org>
Tested-by: Andrew Lamb <andrewlamb@chromium.org>
3 files changed