Update test section in README

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The test scripts assume that travelynx.conf contains a valid database

connection. However, they will override db.user to `travelynx_temp`. This

must be an account which has the same password as the one specified in

travelynx.conf, but uses a temporary schema. This way, tests will always

start from a clean slate and will not leave any trace in the database.

connection. They will create a test-specific schema, perform all operations in

it, and then drop the schema. As such, the database specified in the config is

not affected.

Create this account as follows:

* `createuser -P travelynx_temp` (enter the password you are using for the

  regular development user)

* from inside a psql admin shell: `alter role travelynx_temp set search_path = pg_temp;`

Nevertheless, bugs may happen. Do NOT run tests on your production database.

Please use a separate development database instead.

Run the tests by executing `prove`.