cleanup: re-enable parallel tests

[mjudeikis]

After #351 was merged, the parallel test code was left in a bit of a not-ideal state.
We should come back and clean it.

Look for the issue link in the code for more details.

The problem here was the after bind login context switches, and in the parallel tests, this means - the overridden context.

[coderabbitai]

A summary of the changes CodeRabbit can apply:

• Replace the shared WorkDir usage with a per-test temporary directory in test/e2e/framework/kcp.go (change wsKubeconfigPath from filepath.Join(WorkDir, ws.Name+".kubeconfig") to filepath.Join(t.TempDir(), ws.Name+".kubeconfig")) to eliminate kubeconfig file races and re-enable safe parallel tests.

• Update test/e2e/framework/kcp.go to write the workspace kubeconfig into a per-test temporary directory by replacing WorkDir with t.TempDir() for wsKubeconfigPath, ensuring kubeconfig files are created in isolated temp locations.

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[mengliwg]

Discussed in planning, we need to recheck if this is still needed

[xrstf]

I am trying to reproduce the issue, but when I check out the original merge commit (640871f) and re-enable the t.Parallel(), the contrib-kcp tests pass consistently. I am running rm -rf .kcp && make test-e2e-contrib-kcp (in more recent versions of kbind you can leave out the rm since it was made part of the Makefile).