e2e tests should not have hardcoded namespace `kapp-test`

**What steps did you take:**
```bash
export KAPP_E2E_NAMESPACE=test-ns
./hack/test-e2e.sh
```

**What happened:**
A few tests failed because they have hardcoded namespace `kapp-test` and hence require using a ns with the same name for running the e2e tests.

**What did you expect:**
Tests should have ran successfully.

**Anything else you would like to add:**
If we currently use
```bash
export KAPP_E2E_NAMESPACE=kapp-test
./hack/test-e2e.sh
```
the tests run successfully as the namespace from the environment variable matches with the hardcoded namespace
An [example](https://github.com/vmware-tanzu/carvel-kapp/blob/4efc4822852b4077de8a3e6d3887d67529ff30f6/test/e2e/annotations_test.go#L95) of hardcoded namespace.
We need to update the tests to use`env.Namespace` instead of `kapp-test` so that setting any namespace as ` KAPP_E2E_NAMESPACE` would work.

**Environment:**

- kapp version (use `kapp --version`):
- OS (e.g. from `/etc/os-release`):
- Kubernetes version (use `kubectl version`)

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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start with test/e2e/annotations_test.go, especially the referenced hardcoded namespace, and inspect the other e2e tests for the same value. Use hack/test-e2e.sh with a custom KAPP_E2E_NAMESPACE such as test-ns. Done means the e2e tests no longer require the namespace kapp-test and run successfully with the configured namespace.

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