Add NewRequestWithContext, InvokeWithContext and InvokeFuncWithContext

[tylerslaton]

Instead of breaking existing tests, this PR just adds WithContext variants of the same functions. This allows us to write unit tests like this:

func TestBasic(t *testing.T) {
	harness, input, err := tester.FromDir(managerscheme.Scheme, "testdata/basic")
	if err != nil {
		t.Fatal(err)
	}

	ctx := db.ContextWithPartitionID(context.TODO(), "account-partition-id")
	harness.InvokeFuncWithContext(t, ctx, input, Allocate)
}