The configure option --enable-optimizations enables running test suites to generate data for profiling Python. The resulting python binary has better performance in executing python code. Improvements noted here
From configure help:
--enable-optimizations Enable expensive optimizations (PGO, etc). Disabled by default.
From wikipedia
profile-guided optimisation uses the results of profiling test runs of the instrumented program to optimize the final generated code.
In short, you should not skip tests when using --enable-optimizations as the data required for profiling is generated by running tests.
You can run make -j8 build_all followed by make -j8 install to skip tests once(the tests would still run with install target), but that would defeat the purpose.
You can instead drop the configure flag for better build times.