Task for running (and enforcing passed) tests

Maybe it is an interesting feature to run unit tests either locally before deploying or on production after deploy.

I'm no ruby expert so this was my first approach:

```ruby
namespace :deploy do
#before :deploy, "deploy:run_tests"
before :deploy, "tasks:run_tests"
end

namespace :tasks do

task :run_tests do
run_locally do
execute :php, fetch(:vendor_path) + "/phpunit/phpunit/phpunit --configuration " + fetch(:app_path) + "/phpunit.xml " + fetch(:tests_path)
end
end
```

Based on this question:
* https://stackoverflow.com/questions/6769483/automatically-run-tests-on-deploy-with-capistrano

But I'm not sure how to get a return value to inform cap to continue. Currently this solution will stop deployment even when tests pass.

```
(Backtrace restricted to imported tasks)
cap aborted!
SSHKit::Command::Failed: php exit status: 256
php stdout: PHPUnit 7.2.4 by Sebastian Bergmann and contributors.
```

Trace:
```
/var/lib/gems/2.5.0/gems/sshkit-1.16.1/lib/sshkit/command.rb:99:in `exit_status='
/var/lib/gems/2.5.0/gems/sshkit-1.16.1/lib/sshkit/backends/local.rb:64:in `block in execute_command'
/usr/lib/ruby/2.5.0/open3.rb:205:in `popen_run'
/usr/lib/ruby/2.5.0/open3.rb:95:in `popen3'
/var/lib/gems/2.5.0/gems/sshkit-1.16.1/lib/sshkit/backends/local.rb:46:in `execute_command'
/var/lib/gems/2.5.0/gems/sshkit-1.16.1/lib/sshkit/backends/abstract.rb:141:in `block in create_command_and_execute'
/var/lib/gems/2.5.0/gems/sshkit-1.16.1/lib/sshkit/backends/abstract.rb:141:in `tap'
/var/lib/gems/2.5.0/gems/sshkit-1.16.1/lib/sshkit/backends/abstract.rb:141:in `create_command_and_execute'
/var/lib/gems/2.5.0/gems/sshkit-1.16.1/lib/sshkit/backends/abstract.rb:74:in `execute'
app/config/deploy.rb:70:in `block (3 levels) in '
/var/lib/gems/2.5.0/gems/sshkit-1.16.1/lib/sshkit/backends/abstract.rb:29:in `instance_exec'
/var/lib/gems/2.5.0/gems/sshkit-1.16.1/lib/sshkit/backends/abstract.rb:29:in `run'
/var/lib/gems/2.5.0/gems/capistrano-3.11.0/lib/capistrano/dsl.rb:76:in `run_locally'
app/config/deploy.rb:68:in `block (2 levels) in '
/usr/lib/ruby/vendor_ruby/rake/task.rb:271:in `block in execute'
/usr/lib/ruby/vendor_ruby/rake/task.rb:271:in `each'
/usr/lib/ruby/vendor_ruby/rake/task.rb:271:in `execute'
/var/lib/gems/2.5.0/gems/airbrussh-1.3.0/lib/airbrussh/rake/context.rb:62:in `execute'
/usr/lib/ruby/vendor_ruby/rake/task.rb:213:in `block in invoke_with_call_chain'
/usr/lib/ruby/2.5.0/monitor.rb:226:in `mon_synchronize'
/usr/lib/ruby/vendor_ruby/rake/task.rb:193:in `invoke_with_call_chain'
/usr/lib/ruby/vendor_ruby/rake/task.rb:237:in `block in invoke_prerequisites'
/usr/lib/ruby/vendor_ruby/rake/task.rb:235:in `each'
/usr/lib/ruby/vendor_ruby/rake/task.rb:235:in `invoke_prerequisites'
/usr/lib/ruby/vendor_ruby/rake/task.rb:212:in `block in invoke_with_call_chain'
/usr/lib/ruby/2.5.0/monitor.rb:226:in `mon_synchronize'
/usr/lib/ruby/vendor_ruby/rake/task.rb:193:in `invoke_with_call_chain'
/usr/lib/ruby/vendor_ruby/rake/task.rb:182:in `invoke'
/usr/lib/ruby/vendor_ruby/rake/application.rb:160:in `invoke_task'
/usr/lib/ruby/vendor_ruby/rake/application.rb:116:in `block (2 levels) in top_level'
/usr/lib/ruby/vendor_ruby/rake/application.rb:116:in `each'
/usr/lib/ruby/vendor_ruby/rake/application.rb:116:in `block in top_level'
/usr/lib/ruby/vendor_ruby/rake/application.rb:125:in `run_with_threads'
/usr/lib/ruby/vendor_ruby/rake/application.rb:110:in `top_level'
/usr/lib/ruby/vendor_ruby/rake/application.rb:83:in `block in run'
/usr/lib/ruby/vendor_ruby/rake/application.rb:186:in `standard_exception_handling'
/usr/lib/ruby/vendor_ruby/rake/application.rb:80:in `run'
/var/lib/gems/2.5.0/gems/capistrano-3.11.0/lib/capistrano/application.rb:14:in `run'
/var/lib/gems/2.5.0/gems/capistrano-3.11.0/bin/cap:3:in `'
/usr/local/bin/cap:23:in `load'
/usr/local/bin/cap:23:in `'
Tasks: TOP => deploy => tasks:run_tests
```

Start with app/config/deploy.rb, especially the deploy hook and tasks:run_tests entry point, and reproduce the run_locally PHPUnit command using the versions shown in the report. Done means a passing PHPUnit run allows deployment to continue while a failing run prevents it, with the behavior verified through the deployment task.

Written by the indexing model from the issue text.