fix(jsoncs3): wait for init before asserting the migration timeout

[dschmidt]

New() initializes the metadata storage in a background goroutine. The CleanupStaleShares spec started its 1ms context right after New(), so CleanupStaleShares raced:

if err := m.waitForInit(ctx); err != nil { return err }   // "share manager not yet initialized"
if err := m.waitForMigrations(ctx); err != nil { ... }    // "share manager migrations did not complete"

On a loaded machine the context expires before Init + MakeDirIfNotExist + MarkAllApplied are done, so the first wait returns and the spec fails:

[FAILED] Expected
    : share manager not yet initialized: context deadline exceeded
to contain substring
    : share manager migrations did not complete

Reproduced with 8 busy loops on 4 cores: 21 of 40 runs failed.

This exposes Ready() and waits for it before starting the timeout context. The timeout then only applies to the migration wait, which is what the spec asserts. Same load, 0 of 40 runs failed; package is green under -race.

Seen on https://ci.opencloud.rocks/repos/4/pipeline/586/19, unrelated to the PR it failed on (#790).