Enable SAST spec performance improvement

What does this MR do?

Addresses #325777 (closed)

As part of the review of E2E test MR !56117 (merged) it was identified that performance could be improved by ignoring the status of the MR pipeline as it was not required for the test.

This MR (re)adds a merge_immediately function that either selects the merge_moment_dropdown if found followed by merge_immediately_menu_item, or the merge_button. Note that the logic does not check for merge_button first as this is a container which defaults to merge_when_pipeline_succeeds if the pipeline is still running, negating the point of this change.

Some non-scientific benchmarks against local GDK suggest some improvement against 10x executions. (As an aside, of the logic , When the merge_button is used seems a little slower as it runs a check that it can be merged.)

Baseline before change 10x executions	After change 10x executions
2m 41s	2m 22s

An average improvement of 19 seconds.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Set up GDK or another instance
2. Run test against GDK/instance QA_DEBUG=true CHROME_HEADLESS=false bundle exec bin/qa Test::Instance::All http://<instance_URL>/ ./qa/specs/features/ee/browser_ui/secure/enable_sast_from_configuration_spec.rb

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team