Fix incorrect references to trie in tests

[kn]

In the current master, npm test raises the following error:

TAP version 13
# test the cache api
# should have the correct value in the cache
/Users/kn/repos/ethereumjs-vm/tests/cacheTest.js:65
      vm.trie.put(Buffer.from(account1.address, 'hex'), account.serialize(), cb)
              ^

TypeError: Cannot read property 'put' of undefined

This PR fixes it by correcting references to trie.

[holgerd77]

Thanks for the PR, tests are not actively included in the CI testing atm, but having these up-to-date can't hurt anyhow.

Will directly merge since this is not touching any production (test) code.