add autotools-generated files to .gitignore

When running "make check", Automake generates test-suite.log, along with
various *.log and *.trs files in the tree. Add these files to
.gitignore.

(It looks like this feature arrived in Automake 1.13.)

Signed-off-by: Ken Dreyer <ken.dreyer@inktank.com>
Merge pull request #1042 from kdreyer-inktank/gitignore-autotools-test-logs

add autotools-generated files to .gitignore