This change adds a new test task, platformTest, which runs `gradle test
integTest` within a vagrant host. This will allow us to still test on
all the supported platforms, but be able to standardize on the tools
provided in the host system, for example, with a modern version of git
that can allow #22946.
In order to have sufficient memory and cpu to run the tests, the
vagrantfile has been updated to use 8GB and 4 cpus by default. This can
be customized with the `VAGRANT_MEMORY` and `VAGRANT_CPUS` environment
variables. Also, to save time to show this can work, it currently uses
the same Vagrantfile the packaging tests do. There are a lot of cleanups
we can do to how the gradle-vagrant tasks work, including generating
Vagrantfile altogether, but I think this is fine for now as the same
machines that would run platformTest run packagingTest, and they are
relatively beefy machines, so the higher memory and cpu for them, with
either task, should not be an issue.