Spike: replace libtap with cmocka

[oschwald]

The bundled libtap dependency is unmaintained and has caused build problems with project compiler flags. This spike replaces it with cmocka to evaluate the migration and its failure output.

The changes convert the C tests, remove the libtap submodule, and add cmocka discovery to Autotools and CMake. CMake can fetch cmocka when it is absent. CI dependency setup includes a source build for 32-bit jobs. The four Perl test scripts remain.

This is an exploratory draft. The framework choice is still open, including a small set of internal helpers, Unity, or utest.h.

Known limitations:

• Native 128-bit comparisons use integer assertions that truncate values and can incorrectly pass.
• Assertion wrappers evaluate operands twice, including allocation calls. This can conceal failures.
• CMake accepts older cmocka packages but requires the namespaced target introduced in 2.0.
• Boolean assertions omit actual values. Large test cases lose context and stop at their first failure.
• Replacing the Perl tests still needs evaluation.

Validation: a fresh Linux CMake build with the installed cmocka passed all 29 CTest tests. git diff --check HEAD passed. The current Autotools and cross-platform CI changes have not been validated locally. Passing tests do not resolve the known assertion defects above.

Related: STF-1731, formerly ENG-5416.

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[oschwald]

After trying this out, I think my temptation is to just replace libtap with a small internal helper. Other options to consider are:

• Unity
• utest.h