[P4Testgen] Miscellaneous fixes for P4testgen

[fruffy]

• Unify test numbering and the name of the variable in the test back ends. They all start from 0 now. Also make sure the test name does not have a duplicate ..
• Provide hints on available test back ends.
• Verify that no other packets are present if the packet is dropped in PTF.
• Add some hints on the support Z3 versions. [P4Testgen] Control the Z3 dependency in CMake using fetchcontent, generalize FindZ3.cmake #4034 will fix this, too.
• Support array indices when converting references.

Fixes #4033.

[jafingerhut]

I have tried out these changes and all the changes look like improvements to me. The PTF test generated still has classes named starting at 1, e.g. class Test1 up to class Test10 when generating 10 tests, instead of class Test0 up to class Test9.

[jnfoster]

LGTM