test(kit): add regression test for the posixied instrumentation file

[BastiDood]

Closes #15022.

As a follow-up to #14993, I was able to reliably reproduce the regression. Prior to merging the commit, the emitted output looks like this:

// Actual
import './server\instrumentation.server.js';

// Expected
import './server/instrumentation.server.js';

This PR introduces the regression test that checks against this case specifically.

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[BastiDood]

The test failure is likely flaky and unrelated. I doubt that the new test introduced a regression to the client.test.js script. Should be safe to retry?

1 failed
  [webkit-build] › test/cross-platform/client.test.js:962:2 › Routing › responds to <button formtarget="_blank" submission with new tab 
1 flaky
  [webkit-build] › test/cross-platform/client.test.js:948:2 › Routing › responds to <form target="_blank"> submission with new tab 

[elliott-with-the-longest-name-on-github]

Thank you!