caddyhttp: Fix test when /tmp/etc already exists (#4544)

The TestFileListing test in tplcontext_test has one test that verifies
if directory traversal is not happening. The context root is set to
'/tmp' and then it tries to open '../../../../../etc', which gets
normalized to '/tmp/etc'.

The test then expects an error to be returned, assuming that '/tmp/etc'
does not exist on the system. When it does exist, it results in a test
failure:

```
--- FAIL: TestFileListing (0.00s)
    tplcontext_test.go:422: Test 4: Expected error but had none
    FAIL
    FAIL
    github.com/caddyserver/caddy/v2/modules/caddyhttp/templates	0.042s
```

Instead of using '/tmp' as root, use a dedicated directory created with
`os.MkdirTemp()` instead. That way, we know that the directory is empty.

0 files changed, 0 insertions, 0 deletions