have travis display test-suite.log from the right place when DISTCHECK=yes

After legacy/trac#25814 (moved), Travis CI sometimes won't find test-suite.log because make distcheck runs make check in a subdirectory. This causes problems when there's a failure that's specific to make distcheck, like in this example.

We should create a new make target that shows test-suite.log from the correct place. (It's easier if it's a make target because then we have $(distdir) available.)