[TEST] Expand api singleton test to cover explicit dlopen().

[marcalff]

Fixes # (issue)

Changes

No change in production code, unit test only.

Expand the api singleton test to cover libraries:

• never linked with the main binary at built time
• loaded at runtime with dlopen()

The test case added is only available when building with CMake.

When building with bazel, the test case is commented, due to an existing limitation of bazel.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #2164 (17b95ee) into main (42709f4) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2164   +/-   ##
=======================================
  Coverage   87.51%   87.51%           
=======================================
  Files         169      169           
  Lines        4889     4889           
=======================================
  Hits         4278     4278           
  Misses        611      611           

[marcalff]

Fix works for CMake.

For bazel, blocked by bazelbuild/bazel#4218

[marcalff]

Now ready for review.

[lalitb]

LGTM. Sorry for the delay.