* test_interface
Case-handle expected permissions for newly created private key
files (world-readable on Windows, user-readable elsewhere).

* test_storage
 - Refactor exception tests for better readability.
 - Create test directory and explicitly set permissions, to test
   permission errors more reliably.
 - Skip permission tests on Windows, which does not fully support
   `mode`.

* test_util
Remove permission-error test in ensure_parent_dir method to avoid
platform case handling. This is only an internally used thin
wrapper around create_dirs, which is already tested extensively in
test_storage.

Signed-off-by: Lukas Puehringer <[email protected]>

Prior to this change gpg tests failed because securesystemslib.gpg
interface functions cannot handle absoulte paths as homedir on
Windows. This patch works around the issue to fix tests only by not
using absolute paths:

* Use relative path to homedir for gpg tests, or
* Store and use fixtures of raw pubkey data for gpg parsing tests. These test
don't actually need to run the gpg command. The fixtures were created like so:

```
keyids=(F557D0FF451DEF45372591429EA70BD13D883381 \
        E8AC80C924116DABB51D4B987CB07D6D2C199C7C)
dir=tests/gpg_keyrings/rsa

for keyid in ${keyids[@]}; do
  gpg --homedir ${dir} --export ${keyid} > ${dir}/${keyid}.raw;
done
```

Signed-off-by: Lukas Puehringer <[email protected]>