Allow other test framework than mocha for anchor test.

[fanatid]

I do not like mocha because new global variables introduced so this requires an external runner. Instead, I usually use tape where tests can be run as usual js files. Another good thing for tape that this test framework can be easily pinned (mocha can be pinned too, but we need patch PATH).
Outside tape somebody else can like use jest/ava/etc.

It would be nice if anchor test would accept test framework (through --test-framework=xxx for example) with mocha by default for backward compatibility.

I can work on this feature if you accept it.