Add testsuite book directories to ignore list

This helps if you are creating new tests, or debugging existing tests by
allowing you to run `mdbook` directly in the test directory to try
things out. But we don't want to ever check these files in.

Author: ehuss

.gitignore

@@ -9,6 +9,7 @@ guide/book
 .vscode
 tests/dummy_book/book/
 test_book/book/
+tests/testsuite/*/*/book/
 
 # Ignore Jetbrains specific files.
 .idea/