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.
This will not show the colorful human-readable file results and final
test results summary but instead output a JSON blob containing all test
information, which can then be processed by other programs easily.
Employ the same technique as SpaceAnalyzer to avoid doing full path
resolution in the kernel over an over. Starting each path resolution
from the base of the directory iterator (using its fd) is significantly
faster and reduces test-js runtime by ~3%.
