Split out the functionality to gather multiple tests from the filesystem
and run them in turn into Test::TestRunner, and leave the JavaScript
specific test harness logic in Test::JS::TestRunner and friends.
The TestRunner objects at the end of test-js are destroyed after the
if/else that chooses whether to run the 262 parser tests or the standard
tests. Accessing TestRunner::the() after the lifetime of the TestRunners
ends is UB, so return the Test::Counts from run() instead. Also, fix the
destructor of TestRunner to set s_the to nullptr so that if anyone tries
this type of shenanigains again, they'll get a crash :^).
