speed up wallet tests by three times

The mocha browser tests for Wallet are extremely slow. In this PR I cache a created wallet rather than create the wallet anew for most tests, which mean that a lot of the crypto that would normally get run does not have to run. This makes this tests much faster. On my machine, the tests used to take 163 seconds. With this PR, the same tests take 48 seconds. That saves almost two full minutes from the total run time of the tests.

You can test this by either running all the tests, or just grep for "Wallet" to run only the wallet tests, i.e. index.html?grep=Wallet