add support for apps build with base configured

[mansona]

This PR adds support for apps built with the base config option in Vite

I didn't really know how to test this other than creating a whole other fixture app 🤔 but I have verified this on a consuming app so I know it works fine 👍

[NullVoxPopuli]

a whole new fixture app is the right way -- apps are cheap, and with a monorepo, we can test any configuration situation we can imagine ;) <3

[mansona]

woof that was a tough one to actually test 🫠 there is some heavy interplay between base, outDir, and rootURL 😬

Anyway I have added the test now and verified that it works locally 👍

Edit: I opened a quick PR on my fork to show that this fails without the fix: https://github.com/mansona/testem-code-coverage/actions/runs/36449973913/job/109021919603?pr=1#step:6:45

The output is swallowed by the CI setup but this is the error that I was getting before adding the fix:

[coverage] No app-source coverage data after filtering — falling back to byte-level report.

Coverage Summary (byte-level)
──────────────────────────────────────────────────────────────────────────
File                                                Used B  Total B      %
──────────────────────────────────────────────────────────────────────────
face-face/@embroider/virtual/test-support.js           461      461   100%
face-face/@embroider/virtual/vendor.js                 316      316   100%
face-face/assets/app-DNcO-ENP.js                   2609114  2609114   100%
face-face/assets/tests-BP-Q9J2L.js                  410117   410117   100%
socket.io/socket.io.js                              155835   155835   100%
testem/testem_connection.js                           5692     5692   100%
──────────────────────────────────────────────────────────────────────────
Total                                              3181535  3181535   100%