webpack: Fix regexp for unit test files
webpack: Fix regexp for unit test files

`exclude:` matches on the complete absolute path. This broke when
cockpit was checked out into a directory (e.g. a branch worktree) that
started with "test-".

To avoid that, tighten the regular expression to only match on test-*.js
files.

Fixes #16829

martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit