ci: Use a RAM disk and more CPUs on FreeBSD.

ci: Use a RAM disk and more CPUs on FreeBSD.

Run the tests in a RAM disk.  It's still a UFS file system and is backed
by 20GB of disk, but this avoids a lot of I/O.  Even though we disable
fsync, our tests do a lot of directory manipulations, some of which
force file system meta-data to disk and flush slow device write caches
on UFS.  This was a bottleneck preventing effective scaling beyond 2
CPUs.

Now we can use 4 CPUs like on other OSes, for a huge speedup.

Reviewed-by: Maxim Orlov <orlovmg@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2BFXLcEg1dyTqJjDiNQ8pGom4KrJj4wF38C90thti9dVA%40mail.gmail.com