[scudo][standalone] Fix tests makefile

Summary:
A missing `STATIC` entailed some annoying to debug failures wrt 32 vs 64
binaries. Additionally I noticed I was using the wrong variable (the Scudo
one as opposed to the Scudo Standalone one).

See https://reviews.llvm.org/D58184#1412417 and below for discussion.

Reviewers: vitalybuka, eugenis, brzycki

Reviewed By: vitalybuka, brzycki

Subscribers: mgorny, delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D58794

llvm-svn: 355203