Refactor ForkService tests

What does this MR do and why?

Contributes to #346563 (closed)

Problem

Tests for ForkService depend on ProjectForksHelper that applies applies multiple modifications to the state of objects. There is no guarantee that ForkService works as expected.

Solution

Don't use fork_project method from ProjectForksHelper for ForkService tests.

Performance

Before

Finished in 1 minute 7.23 seconds (files took 12.28 seconds to load)
57 examples, 0 failures

After

Finished in 29.4 seconds (files took 12.74 seconds to load)
44 examples, 0 failures

New version is more than twice as fast comparing to the current version. 🚀