Prefix task types used in tests with 'test-'

so that tests do not depend on a lucky guess on what the scheduler db
state actually is. DB initialization scripts do create task types for
git, hg and svn (used in tests) but these tests depends on the fact the
db fixture has been called already once before, so tables are
truncated (especially the task and task_type ones).

For example running a single test involved in task-type creation was
failing (eg. 'pytest swh -k test_create_task_type_idempotence').

This commit does make tests not collide with any existing task or task
type initialization scripts may create.

Note that this also means that there is actually no test dealing with
the scheduler db state after initialization, which is not grat and
should be addressed.