DexMerger: Workaround for buffer overflow in tests

Adds a fudge factor to the estimated debug info size. The existing
estimation breaksdown with small dex files on the buildbots. The use
of estimation is fast, but error prone.

Re-enables 127-merger-stress.

Bug: 68483205
Test: dalvik/dx/tests/run-all-tests
Change-Id: I4c836721af336625b2fa2972c9599f4c9c8dd804