Allow explicit schema injection to rivertest.Require* test functions

[brandur]

Here, resolve #907 by letting an explicit schema be injected into
rivertest.Require* assertions in a similar way that one can be used in
a client.

This approach adds a schema in RequireInsertedOpts. This comment does
a good job of highlight all the potential approaches for adding a schema
[1], and unfortunately none of them are all that great. I implemented
one other version of this (a variant of option 2 in that list), which as
some advantages, but in the end it just ended up ballooning the API out
to an uncomfortable degree.

The worst part about adding schema to RequireInsertedOpts is its
interact with the RequireMany* functions, where each expectation can
set its own schema, and it's not clear what would happen if different
expectations set different schemas. I resolved this ambiguity by making
it an error to mix and match schemas. Assertions are allowed to send a
schema in only the first position like:

jobs := requireManyInserted(ctx, bundle.mockT, bundle.driver, []ExpectedJob{
    {Args: &Job1Args{String: "foo"}, Opts: bundle.schemaOpts},
    {Args: &Job1Args{String: "bar"}},
})

Or send the same schema in all positions:

jobs := requireManyInserted(ctx, bundle.mockT, bundle.driver, []ExpectedJob{
    {Args: &Job1Args{String: "foo"}, Opts: bundle.schemaOpts},
    {Args: &Job1Args{String: "bar"}, Opts: bundle.schemaOpts},
})

But they aren't allowed to set a schema only in position other than the
first, or mix and match schemas between expectations.

Fixes #907.

[1] #907 (comment)

[bgentry]

I think this seems like the best of the possible options and you explored those pretty thoroughly. Great pragmatic choice!

[brandur]

+1, wish there was a better option, but this should do it for now. Thanks!