Make it easy to run the test suite with a ramdisk mounted on its temporary directory

add a --temp-dir-ramdisk [SIZE] option
- the default value for SIZE would be whatever is needed by a full run of the test suite
- if/when/once the required ramdisk size grows larger than what we can allocate on our isotesterN VMs, we’ll add swap and let the kernel deal with spreading data among the memory and disk; we’ll probably want to add a couple SSDs in RAID-0 to host these swap volumes, by the way

Original created by @intrigeri on 8681 (Redmine)

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