feat: Improve failure mode of missing method test

[MichaelChirico]

Currently this can fail like:

hasMethod("dbDisconnect", "MyDBIConnection", driver_package) is not TRUE

`actual`:   FALSE
`expected`: TRUE 
Backtrace:
     x
  1. \-DBItest:::spec_compliance$compliance(ctx = ctx)
  2.   \-base::sapply(...) at [rbuild/R/spec-compliance-methods.R:30]
  3.     \-base::lapply(X = X, FUN = FUN, ...)
  4.       \-DBItest (local) FUN(X[[i]], ...)
  5.         \-base::mapply(...) at [rbuild/R/spec-compliance-methods.R:44]
  6.           \-DBItest (local) `<fn>`(dots[[1L]][[1L]], dots[[2L]][[1L]])
  7.             \-DBItest:::expect_has_class_method(method, class, args, where) at [rbuild/R/spec-compliance-methods.R:45]
  8.               +-base::eval(...) at [rbuild/R/spec-compliance-methods.R:100]
  9.               | \-base::eval(...)
 10.               \-testthat::expect_true(...)

I'm unsure why this is failing, but seeing driver_package makes it a bit harder -- with this change the error should look like this instead:

hasMethod("dbDisconnect", signature = "MyDBIConnection", asNamespace("<searched package>")) is not TRUE

That will give something more actionable for my debugging.

[krlmlr]

Thanks. I'm fine if it helps you, didn't look too closely. Let's see what happens with the other backends.