[P4Testgen] Introduce a configuration structure for the test back ends.

[fruffy]

This pull request adds the TestBackendConfiguration object to P4Testgen. This object contains configuration options which are consumed by the test back ends. Most of the changes involve refactoring.

Currently, the object has the testname, seed, the test path and the amount of tests to generate as inputs.

[vlstill]

It would be great to lift this into a lib function (of the config?), parametrized by the test id and extension.

[fruffy]

It's an annoyingly common pattern with std::optional and std::expected. Makes you wish C++ had a ? operator.

I have been thinking about introducing https://github.com/TartanLlama/optional at some point, since it fills the gap std::optional left with optional references and unpacking.
Or simply use some custom macro like this

#define ASSIGN_OR_RETURN_WITH_MESSAGE_IMPL(temporary, targetVariable, inputFunction, returnValue, \
                                           errorFunction)                                         \
    auto(temporary) = inputFunction;                                                              \
    if (!(temporary)) {                                                                           \
        errorFunction;                                                                            \
        return returnValue;                                                                       \
    }                                                                                             \
    targetVariable = *(temporary);  // NOLINT

#define ASSIGN_OR_RETURN_WITH_MESSAGE(targetVariable, inputFunction, returnValue, errorFunction) \
    ASSIGN_OR_RETURN_WITH_MESSAGE_IMPL(STATUS_MACROS_CONCAT_NAME(temporary, __LINE__),           \
                                       targetVariable, inputFunction, returnValue, errorFunction)

There is also https://github.com/abseil/abseil-cpp/blob/master/absl/status/statusor.h.

[fruffy]

I have created an issue to track this #4403.

[vlstill]

Yes, the general case is interesting too, but here I meant the function to derive test file name from configuration, which is almost exactly the same in most testgen backend.

[fruffy]

Yes, there are more patterns that are similar in the test back ends. Any back end that writes to files and does not use a preamble (PTF) has a similar pattern. I could move the entire sequence into a helper.

However, I would like to clean up the error handling first before introducing a new utility function.