Add unit tests for $$ appearing within policy inputs

[michel-laterman]

What does this PR do?

Add unit tests for special characters, mainly the escape sequence of $$ in the transpiler, and input parsing of policies.

Why is it important?

Secrets are being incorrectly formatted when they contain these characters.
This also applies to any key: value pair the user specifies in the fleet-ui.

Testing found that this is caused by the UI escaping $$ to $; but we should have unit tests in the agent to ensure we don't inadvertently introduce a regression.

Related issues

• Relates all: secrets cannot contain $$ or ${SOMETEXT} kibana#188377

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[pierrehilbert]

Skipping the changelog check as this PR is only adding Unit Tests