Switch to terser for tests as well

[pravi]

Expected behavior

sprockets should test with terser gem instead of uglifier as this is the recommended option. See #713

Actual behavior

uglifier is still used in tests https://github.com/rails/sprockets/blob/master/sprockets.gemspec#L35

System configuration

• Sprockets version
• Ruby version

Example App (Reproduction) - THIS IS IMPORTANT YOUR ISSUE LIKELY WILL NOT BE RESOLVED WITHOUT THIS

[rafaelfranca]

There is no terser compressor inside the sprockets codebase, but there is a uglifier compressor inside sprockets. That is why we are testing with uglifier but not terser. Since we don't plan to include the terser compressor in the sprockets codebase, and the terser compressor is already being tested in the terser gem, I think it doesn't make sense to test this again.

Thank you for the suggestion though.