2. Tests and ERB

[thisduck]

This PR does the following:

1. Makes tests runnable via bundle exec ruby -Ilib:test -e 'ARGV.each { |f| require f }' ./test/test*.rb
2. Coverts flamegraph.html to flamegraph.html.erb, using ERB to load the html.
3. Uses a TEST environment variable to be able to loan a data.json file for testing in development.