[Lang] Add a test case involving bit-vector concatenation

[ngernest]

Right now our DSL has a bit vector concatenation operator, but it seems none of our example .prot files use it. It'd be helpful if we had an example protocol that used this operator (just to make sure its implemented properly)

[Nikil-Shyamsunder]

https://github.com/ekiwi/transactional-verification-with-protocols/blob/039fb8d5beafb3c4a464e5dac3dc8a3e00b69f04/picorv32/picorv32_pcpi_mul.py#L17C4-L17C14

the old pcpi mul example does use the concatenation, mainly just as a convenience to decompose the RISC-V instruction