Defer registration of singletons and pre-singletons in TestContext

In preparation for allowing the TestContext to enforce an ordering on
the registration of singletons and pre-singletons in the same way as it
does for mutators this defers the registration of them into the
underlying Context.

Bug: 181953909
Test: m nothing
Change-Id: I2d9652122bb6387b6b47ca4761e811885d15c2b6