Increase Capybara wait time for few DAST specs

What does this MR do and why?

Increases Capybara wait time for assertions in ee/spec/features/projects/security/user_creates_on_demand_scan_spec.rb

The test appears to be flaky with the error:

Timeout (30s) reached while running a waiting Capybara finder.
            Consider using a non-waiting finder.

Following up with investigation on: #458216 (closed)

Examples:

1. gitlab-org/release/tasks#10317 (closed)
2. gitlab-org/release/tasks#10311 (closed)
3. gitlab-org/quality/engineering-productivity/master-broken-incidents#6074 (closed)

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