Actually make collections PRS enabled in tests

[HoustonPutman]

Fixes issues with #2230, where collections didn't have the PRS flag enabled when the default was enabled.

This does break tests, so it can't be merged without fixing the issues with PRS.

[dsmiley]

So the change is to non-test code but it's actually about tests? Would it be more accurate to say that there was a partial/broken attempt to make a cluster-wide default for PRS but this PR completes that?

nit: assuming this system property isn't really advertised/documented, I'd rather it be "solr.cloud.prs" (maybe ".enabled" suffix) and not have the word "default" in it. No sys props needs "default" as a system prop by its very nature is a kind of default.

[HoustonPutman]

It is not advertised at all, I believe it is just used for testing. But I think default in the name is fine. Solr.cloud.prs=true, doesn't really tell me what I'm setting to true. Maybe enabled, but that sounds like you are allowing/disallowing something. Maybe we put "test" in the name so that it's clear this is just for testing?

[dsmiley]

Even if today we only use it for testing (because only we know of its secret existence), isn't the same mechanism appropriate for a cluster? It needn't be the only way to set it, of course; I could imagine additional layers like cluster props or solr.xml.

solr.cloud.prs.enabled then