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.

BUG=b:283907263
TEST=CQ
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 <[email protected]>
Commit-Queue: Chris Gerber <[email protected]>
Auto-Submit: Andrew Lamb <[email protected]>
Tested-by: Andrew Lamb <[email protected]>
3 files changed