Reduce the included core-js polyfills

[core-js polyfills](https://github.com/bitovi/testee-client/blob/master/src/index.js#L1) are included for many things when we probably only need a couple:


Start with the core-js polyfill imports at the top of src/index.js, as identified in the issue. Determine which included polyfills are actually needed by the client adapters and reduce the set to those few; verify that the resulting client bundle still works with the supported adapters.

Written by the indexing model from the issue text.