Tests with testscript

Like the tests in package ts, also the ones here use testscript, with a notable difference: they don't use real database servers, and in fact can run even in places without the possibility of downloading them.

A large portion of the tests use a mocking recipe. It is similar to what is done in the sandbox package, but much more resilient here, as it uses the txtar capabilities of testscript to generate fake database binaries while also isolating the testing environment. When the fake database binaries are created, dbdeployer will use them as if they were real ones.