ICU-22781 Add test for portion format with parts per billion (C++)

[younies]

Description:

• Added a new test method TestPortionFormat() in numbertest_api.cpp
• Updated numbertest.h to declare the new test method
• Test covers formatting of "portion-per-1e9" unit in different locales
• Includes test cases for singular and plural forms
• Handles known issue for other portion units with a log message

java code : #3390

Checklist

• Required: Issue filed: ICU-22781
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[jira-pull-request-webhook]

Hooray! The files in the branch are the same across the force-push. 😃

~ Your Friendly Jira-GitHub PR Checker Bot

[richgillam]

Test case looks good, but you might want to put comments on something on all the UNKNOWN things that are covered by the logKnownIssue() in the code.

[richgillam]

All of these UNKNOWN things are being caught by the logKnownIssue() below, right? You should probably put comments on the test cases themselves indicating this too.

[younies]

yes, all of them are being caught by logKnownIssue,

Okay, I will add a comment in a following PR.