Add env to skip mlock test

[mafintosh]

When running on my virtual host this test always fails (there might be more), due to a missing capability.

/root/sodium-native/test/memory.js:24
  sodium.sodium_mlock(buf)
         ^

Error: EPERM, Operation not permitted
    at Test.<anonymous> (/root/sodium-native/test/memory.js:24:10)
    at Test.bound [as _cb] (/root/sodium-native/node_modules/tape/lib/test.js:77:32)
    at Test.run (/root/sodium-native/node_modules/tape/lib/test.js:96:10)
    at Test.bound [as run] (/root/sodium-native/node_modules/tape/lib/test.js:77:32)
    at Immediate.next (/root/sodium-native/node_modules/tape/lib/results.js:75:19)
    at runCallback (timers.js:705:18)
    at tryOnImmediate (timers.js:676:5)
    at processImmediate (timers.js:658:5)

Would be nice if we had a flag to disable those to still run the full test suite on systems like that.