Merge branch 'preserve-gem-path' into 'master'

Preserve gem path so that we use the same gems

See merge request gitlab-org/gitlab-ce!15986

scripts/gitaly-test-spawn

 #!/usr/bin/env ruby
 
 gitaly_dir = 'tmp/tests/gitaly'
-env = { 'HOME' => File.expand_path('tmp/tests') }
+env = { 'HOME' => File.expand_path('tmp/tests'),
+        'GEM_PATH' => Gem.path.join(':') }
 args = %W[#{gitaly_dir}/gitaly #{gitaly_dir}/config.toml]
 
 # Print the PID of the spawned process