The new function should be used here. Because the entry needs to be reused, that call for a slightly different API.

This name is not very good. It needs to convey what is cached, so what it returns is understood.

What does CheckCache means ? That the cache in a self consistent state ? That the entry we are looking for is in the cache ?

There is nothing called GetSigCache. What about simply TestCachingTransactionSignatureChecker?

Include standard lib last

Relayout. Aggregate the string pieces in one big string and let clang-format do its job.

You may want to check that no entry is inserted into the cache here.

This still needs to use the same code as above, or you are testing a piece of code while running another one, which is not very useful.

There are many option to achieve this. For isntance you can have a wrapper like

template<typename F> RunMemoizedCheck(const std::vector<uint8_t> &vchSig, const CPubKey &pubkey,
    const uint256 &sighash, const F& fun) {
    if (signatureCache.Get(entry, !store)) {
        return true;
    }
    if (!fun()) {
        return false;
    }
    if (store) {
        signatureCache.Set(entry);
    }
    return true;
}

bool CachingTransactionSignatureChecker::VerifySignature(
    const std::vector<uint8_t> &vchSig, const CPubKey &pubkey,
    const uint256 &sighash) const {
    return RunMemoizedCheck(vchSig, pubkey, sighash, [&]{
         return TransactionSignatureChecker::VerifySignature(vchSig, pubkey, sighash);
    });
}

bool CachingTransactionSignatureChecker::IsCached(
    const std::vector<uint8_t> &vchSig, const CPubKey &pubkey,
    const uint256 &sighash) const {
    return RunMemoizedCheck(vchSig, pubkey, sighash, []{ return false; });
}

There are many options.

Store is a very bad name. This controls whether thing are stored in the cache, for sure, but this is also to mark an entry as to be cleaned or not. This needs a better name.

Arguably, the checker shouldn't be a member here. It doesn't make a lot fo sense This class's responsability is to provide an interface to features of CachingTransactionSignatureChecker which are not publicly accessible for testing purposes.

Parsing transaction and all are clearly out of scope.