gems: don't load listen and solargraph in test mode.

Only load the `listen` and `solargraph` gems in the development
environment, not the test environment. The `listen` gem automatically
spawns background threads to listen for file changes, in order to
automatically reload code when files change, which we don't want or need
in test mode. These threads can interfere with sandboxing, because they
prevent us from being able to call unshare(2) (which can only be called
from a single-threaded process).

Gemfile

@@ -67,9 +67,6 @@ group :development do
   gem 'better_errors'
   gem 'binding_of_caller'
   gem 'benchmark-ips', require: "benchmark/ips"
-end
-
-group :development, :test do
   gem 'listen'
   gem 'solargraph'
 end