Fix the nasty testing hack

[KyleMaas]

Why does this need to be here, and what would it take to eliminate it?

margaret/indexes/badger/index.go

Line 32 in 22ed48a

// NASTY TESTING HACK

It pops up in every (properly built) testing run of go-ssb.

[KyleMaas]

At the very least, this should be turned into a DisableWriteCache() call or something similar rather than relying on an environmental variable.

[KyleMaas]

And to clarify: I'm not asking what this does. I can see it tries to make writes fully synchronous so they're not cached. But if this system's buggy enough that go-ssb's testing suite needs this, and thus I assume all other downstream users of Margaret would need it to be reliably tested as well, then what bugs exist in the write queue and how do we fix them? That's what I mean by "why does this need to be here".