[test][fuzzer] Use %python for trace-malloc-unbalanced

[gbfarlej]

Summary

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.

compiler-rt/test/fuzzer/lit.cfg.py

if sys.platform.startswith('linux'):
   config.available_features.add('linux')
 else:
   lit_config.note('linux feature unavailable')
 
 config.substitutions.append(('%build_dir', config.cmake_binary_dir))
 libfuzzer_src_root = os.path.join(config.compiler_rt_src_root, "lib", "fuzzer")
 config.substitutions.append(('%libfuzzer_src', libfuzzer_src_root))
 
+config.substitutions.append(('%python', '"%s"' % (sys.executable)))
+
 def generate_compiler_cmd(is_cpp=True, fuzzer_enabled=True, msan_enabled=False):
   compiler_cmd = config.clang
   extra_cmd = config.target_flags
 
   if is_cpp:
     std_cmd = '--driver-mode=g++'
   else:
     std_cmd = ''

compiler-rt/test/fuzzer/trace-malloc-unbalanced.test

 // FIXME: This test infinite loops on darwin because it crashes
 // printing a stack trace repeatedly
 UNSUPPORTED: darwin
 
 // Verifies lib/fuzzer/scripts/unbalanced_allocs.py script
 
 RUN: %cpp_compiler %S/TraceMallocTest.cpp -o %t-TraceMallocTest
 
 # Specify python because we can't use the shebang line on Windows.
 RUN: %run %t-TraceMallocTest -seed=1 -trace_malloc=1 -runs=200 2>&1 | \
-RUN:    python %libfuzzer_src/scripts/unbalanced_allocs.py --skip=5 | FileCheck %s
+RUN:    %python %libfuzzer_src/scripts/unbalanced_allocs.py --skip=5 | FileCheck %s
 
 RUN: %run %t-TraceMallocTest -seed=1 -trace_malloc=2 -runs=200 2>&1 | \
-RUN:    python %libfuzzer_src/scripts/unbalanced_allocs.py --skip=5 | FileCheck %s --check-prefixes=CHECK,CHECK2
+RUN:    %python %libfuzzer_src/scripts/unbalanced_allocs.py --skip=5 | FileCheck %s --check-prefixes=CHECK,CHECK2
 
 CHECK: MallocFreeTracer: START
 # Behavior of the format string "%p" is implementation defined. Account for the
 # implementation on Windows and Linux.
 CHECK: Unbalanced MALLOC[{{[0-9]+}}] [[PTR:(:?0x)?[0-9a-fA-F]+]] 4
 CHECK2-NEXT: {{ #0 0x[0-9a-f]+ in }}
 CHECK2-NEXT: {{ #1 0x[0-9a-f]+ in }}
 CHECK2-NEXT: {{ #2 0x[0-9a-f]+ in }}
 CHECK: MallocFreeTracer: STOP
 
 CHECK: MallocFreeTracer: START
 CHECK: Unbalanced FREE[{{[0-9]+}}] [[PTR]]
 CHECK2-NEXT: {{ #0 0x[0-9a-f]+ in }}
 CHECK2-NEXT: {{ #1 0x[0-9a-f]+ in }}
 CHECK2-NEXT: {{ #2 0x[0-9a-f]+ in }}
 CHECK: MallocFreeTracer: STOP