update retry_count to get the number from a var or use default 0 (#397)

When running locally we might not want the tests to retry, to do this
we would need to comment out the retry lines. We want to make this easier
on the developer

Converting the env variable to an integer because ENV[] on ruby returns a string
https://ruby-doc.org/core-3.0.0/ENV.html

.travis.yml

@@ -10,8 +10,8 @@ before_install:
 before_script:
 - docker-compose exec pender test/setup-parallel
 script:
-- docker-compose exec pender bundle exec rake "parallel:test[3]"
-- docker-compose exec pender bundle exec rake parallel:spec
+- docker-compose exec -e TEST_RETRY_COUNT=5 pender bundle exec rake "parallel:test[3]"
+- docker-compose exec -e TEST_RETRY_COUNT=5 pender bundle exec rake parallel:spec
 after_script:
 - docker-compose exec pender cat tmp/performance.csv
 - docker-compose exec -e TRAVIS_PULL_REQUEST=$TRAVIS_PULL_REQUEST -e TRAVIS_TEST_RESULT=$TRAVIS_TEST_RESULT -e TRAVIS_BRANCH=$TRAVIS_BRANCH -e CC_TEST_REPORTER_ID=$CC_TEST_REPORTER_ID -e GIT_COMMIT_SHA=$GIT_COMMIT_SHA -e GIT_COMMITTED_AT=$GIT_COMMITTED_AT pender test/test-coverage

test/test_helper.rb

@@ -12,7 +12,7 @@
 require 'minitest/mock'
 require 'mocha/minitest'
 
-Minitest::Retry.use!(retry_count: 5)
+Minitest::Retry.use!(retry_count: ENV['TEST_RETRY_COUNT'].to_i || 0)
 
 Minitest::Retry.on_failure do |_klass, _test_name|
   sleep 10