Align 3.6 test helpers with development

[davidhorstmann-arm]

Align the tests/src and tests/include directories in development with mbedtls-3.6 in preparation to move these to the framework.

After this PR and its forward-port #9430 are merged, the tests/src and tests/include directories should be identical in development and mbedtls-3.6, except for the following 2 cases:

• tests/src/psa_test_wrappers.c and tests/include/test/psa_test_wrappers.h differ because these files are generated based on the supported PSA featureset. These will not be moved to the framework, at least for the time being.
• Headers in tests/include/alt-dummy/* are related to the 3.6-only _ALT interface, so will not be moved to the framework and are only present in 3.6.

PR checklist

Please remove the segment/s on either side of the | symbol as appropriate, and add any relevant link/s to the end of the line. If the provided content is part of the present PR remove the # symbol.

• [x] changelog not required because: No user-facing changes
• [x] development PR provided #9430
• [x] framework PR not required - no framework changes
• [x] 3.6 PR provided #9547
• [x] 2.28 PR not required because: No framework in 2.28 so no test helper alignment required.
• tests not required because: Internal changes only, existing tests suffice.