Add check for generated curl command in integ test

[fabricereix]

In verbose mode, Hurl writes to stderr the curl commands to be executed to produce the same requests.

Requirements in order to apply the check for the generated curl commands:

• test run in verbose mode
• existence of an expected .curl file

[jcamiel]

See #2679

[jcamiel]

@fabricereix --curl is available now, I've added two integrations tests tests_ok/assert_body_curl.sh and tests_ok/multilines_curl.sh. Don't know yet if we use the current integration tests or if we do dedicated tests with tests_curl/*. I like this simple first step, it fits in our current integration tests model, we can reuse existing Flask endpoints, we can add curl integration tests to tests_failed/*etc... To discuss !