make detect_leaks=1 the default for the lsan test suite

Summary:
This already appears to be the case in all .cc test files,
it was probably left out of the .c test files accidentally. Make it a global
default, instead of manually adding it to each individual test.

This is needed to force leak detection for Darwin tests, where leak detection
is disabled by default.

Reviewers: m.ostapenko, kubamracek, alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32297

llvm-svn: 300890