simplify running unit tests

[mrothauer]

in database.php the $default config needs to be set to $test config before running unit tests - as most of them are integration tests with database access and only a few are real unit tests.

task
manage somehow that the db config is changed automatically as soon as unit tests are run - and change it back as well

hints
a lot of tests work with "simple browser" calls. simple browser is a php browser. changing the database always on simple test calls is not enough, because some cron jobs (order lists, invoicesd) also work with simple browser. maybe attaching a header in test context will be a solution worth trying

[mrothauer]

#57