test: don't clobber RegExp.$_ on startup
Some tests expect it to be empty so clear it again after running a regular expression against /proc/cpuinfo.
It didn't cause test failures on any of the CI machines, to the best of my knowledge, because most of the time /proc/cpuinfo doesn't contain the string it was looking for.
Fixes: https://github.com/nodejs/node/issues/44840
A more general solution would be to put it at the bottom of test/common/index.js but there's only one regular expression now so