[Slim] Generation of UnitTest stubs

Created by: ybelenko

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

PHPUnit4 documentation

1. PHPUnit4.8 added to Composer dev dependencies.
2. PHPUnit config file created and script called test added to Composer.
3. Model and Api test templates created. It's just stubs for now without any assertions.
4. All samples has been refreshed.

Test script output below:

$ composer test
Deprecated: The each() function is deprecated. This message will be suppressed on further calls in C:\wamp64\www\openapi-generator\samples\server\petstore\php-slim\vendor\phpunit\phpunit\src\Util\Getopt.php on line 38
PHPUnit 4.8.36 by Sebastian Bergmann and contributors.

..................................

Time: 490 ms, Memory: 4.00MB

OK (34 tests, 0 assertions)
> phpunit --testsuite Models

Deprecated: The each() function is deprecated. This message will be suppressed on further calls in C:\wamp64\www\openapi-generator\samples\server\petstore\php-slim\vendor\phpunit\phpunit\src\Util\Getopt.php on line 38
PHPUnit 4.8.36 by Sebastian Bergmann and contributors.

...............................................................  63 / 133 ( 47%)
............................................................... 126 / 133 ( 94%)
.......

Time: 229 ms, Memory: 6.00MB

OK (133 tests, 0 assertions)

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh