Improved in-test config management

Reduces code duplication, improves tests structuring.

Previous code:

clear_config_all([:instance, :federation_reachability_timeout_days]) do
  Pleroma.Config.put([:instance, :federation_reachability_timeout_days], 1)
end

clear_config([:restrict_unauthenticated, :activities, :local]) do
Pleroma.Config.put([:restrict_unauthenticated, :activities, :local], true)
end

# When we needed to adjust config for a single test case
describe "(had to create context)" do
  clear_config(:x) do: Pleroma.Config.put(:x, 5)

  test "does what it does with x = 5" do
    # ...
  end
end

With this MR:

setup_all do: clear_config([:instance, :federation_reachability_timeout_days], 1)

setup do: clear_config([:restrict_unauthenticated, :activities, :local], true)

test "does what it does with x = 5" do
  clear_config(:x, 5)
  # ...
end

[IMPLEMENTED] What we could do in future is switching to setup do: clear_config(..., ...) in place of clear_config(..., ...). This way we could use the same macro for in-test and setup-level invocation (right now in-test function has to be separate, named it set_config/2). If this switch sounds good, let me know. Pro: same function name, contra: prepending all setup-level calls with setup do:.