benchmark: add default configs to buffer benchmark

Add default values to use for `type` and `method` in `buffer` benchmarks
when the provided configuration value is an empty string. This is
primarily useful for testing, so the test can request a single iteration
without having to worry about providing different valid values for the
different benchmarks.

While making this change, some `var` instances in immediately
surrounding code were changed to `const`. In some cases, `var` had been
preserved so that the benchmarks would continue to run in versions of
Node.js prior to 4.0.0. However, now that `const` has been introduced
into the benchmark `common` module, the benchmarks will no longer run
with those versions of Node.js anyway.

PR-URL: https://github.com/nodejs/node/pull/15175


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>