Riscof picks tests that are not supported

[marcfedorow]

In particular, if your core supports Zkt but doesn't support Zbkx, you may see the following error message:

ERROR | Error in test: /bear/tests/riscv-arch-test/riscv-test-suite/rv32i_m/K/src/zip-01.S
Test Selected without the relevant extensions being available on DUT

This is because of inadequate logic that checks for extension support.
E.g. it checks if zk is supported by checking if isa-string contains zk. The string rv..._zkt_... does contain _zk so it thinks that zk-ext is supported. Thus it thinks that any sub-extension of Zk-ext is supported.
Zkt does not guarantee that any (other) Zk-subextension is implemented.

Vendors do not have to implement all of the list’s instructions to be Zkt compliant; however, if they claim to
have Zkt and implement any of the listed instructions, it must have data-independent latency.
For example, many simple RV32I and RV64I cores (without Multiply, Compressed, Bitmanip, or Cryptographic
extensions) are technically compliant with Zkt

Thus it is wrong to imply that if Zkt is supported then e.g. Zbkx is supported.

This is a general issue with the isa-string parser, not directly related to Zk-ext.