[devtools] Import test runners in http/tests/devtools/resource-tree

This CL replaces 'TestRunner.loadTestModule' calls with import
statements.

The CL was fully auto-generated by a small regex search&replace script.

R=wolfi@chromium.org

Bug: 1442410
Change-Id: Iac7d1fa4c30d7948bc26099cd211b2920dc46920
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4547640


Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Wolfgang Beyer <wolfi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147666}