Fix KNOWN_VALGRIND_TEST_FAILURES.

Tests were re-added to this list in 301a38a5a9d9 without restoring
the backslash-escape for the newline after the +=, effectively
ignoring the rest of the list. We didn't notice previously because
all the valgrind failures were also XFAILS.

Fixes issue #88.
Merge pull request #91 from rillian/dev-fixes

Fix KNOWN_VALGRIND_TEST_FAILURES.