Add testing.M / TestMain support that ensures a DB connection can be made

[dhermes]

It may be nice to utilize https://github.com/ory/dockertest as well, but this seems very heavyweight.

[dhermes]

A common pattern with https://godoc.org/github.com/blend/go-sdk/db is to do

func TestMain(m *testing.M) {
	conn, err := db.New(db.OptConfigFromEnv())
	if err != nil {
		log.Fatalf("%+v", err)
	}
	err = OpenDefaultDB(conn)
	if err != nil {
		log.Fatalf("%+v", err)
	}
	assert.Main(m)
}

so it may be worth one (or both) of

• supporting mapping environment variables to a postgres.Config object (e.g. via a hard-coded path helper or via a struct tag)
• adopting go-sdk/db helpers here (I was trying to avoid having dependencies outside of the standard library; even driver dependencies are not included)

---

Subscript, see current dependencies:

$ git log -1 --pretty=%H
1f7dffc658eb39b0c473c107bafeaac66168d9e8
$ go list -f '{{ .ImportPath }} | {{ .Imports }}' ./...
github.com/dhermes/golembic | [context database/sql errors fmt io/ioutil sync time]
github.com/dhermes/golembic/command | [context errors fmt github.com/dhermes/golembic github.com/dhermes/golembic/postgres github.com/spf13/cobra github.com/spf13/pflag os strings time]
github.com/dhermes/golembic/examples | [github.com/dhermes/golembic path/filepath]
github.com/dhermes/golembic/examples/cmd | [github.com/dhermes/golembic/command github.com/dhermes/golembic/examples github.com/lib/pq log]
github.com/dhermes/golembic/examples/script | [context github.com/dhermes/golembic github.com/dhermes/golembic/examples github.com/dhermes/golembic/postgres github.com/lib/pq log os]
github.com/dhermes/golembic/postgres | [database/sql errors fmt github.com/dhermes/golembic net/url strings time]