benchmark: provide default methods for assert

The benchmarks for `assert` all take a `method` configuration option,
but the allowable values are different across the files. For each
benchmark, provide an arbitrary default if `method` is set to an empty
string. This allows all the `assert` benchmarks to be run with a single
command but only on a single method. This is primarily useful for
testing that the assert benchmark files don't contain egregious errors.
(In other words, it's useful for testing.)

PR-URL: nodejs/node#15174
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: Trott