[Fuzzer][Test] Use %python substitution for trace-malloc-unbalanced.test

This test was found to fail for some of our downstream builds, on
computers where python was not on the default $PATH. Therefore
add a %python substitution to use sys.executable, based on similar
solutions for python calls in tests elsewhere in LLVM.

Differential Revision: https://reviews.llvm.org/D96799

GitOrigin-RevId: f8ed31cd991bf84372369409595707f6e3cebbce
2 files changed