Add createst script

Add createst script

createst is a script to produce a test directory with test.yaml as per
the PCAP and configuration provided.

This currently implements the functionality of creating the "checks" block
in `test.yaml` from a given `eve.json`. You can add other configuration
in the file thus created.

Usage
=====
```
usage: createst.py [-h] [--output-path <output-path>] [--eventtype-only]
                   [--allow-events [ALLOW_EVENTS]]
                   <test-name> <pcap-file>

Create tests with a given PCAP. Execute the script from a valid Suricata
source directory.

positional arguments:
  <test-name>           Name of the test folder
  <pcap-file>           Path to the PCAP file

optional arguments:
  -h, --help            show this help message and exit
  --output-path <output-path>
                        Path to the folder where generated test.yaml should be
                        put
  --eventtype-only      Create filter blocks based on event types only
  --allow-events [ALLOW_EVENTS]
                        Create filter blocks for the specified events
```