Highlight Bats test files as Shell

Bats (https://github.com/sstephenson/bats) test files are Bash with a little extra syntax on top. GitHub's Bash highlighting handles it just fine.

Some example files:
https://github.com/sstephenson/rbenv/blob/master/test/rbenv.bats
https://github.com/sstephenson/ruby-build/blob/master/test/mirror.bats
https://github.com/onehealth-cookbooks/sysctl/blob/master/test/integration/default/bats/attrs.bats
https://github.com/telemachus/quotecat/blob/master/test/runner.bash