[lit] pass LLVM_SYMBOLIZER_PATH through to tests.

Currently several buildbots give unsymbolized traces on crash.
I suspect these are configuring the symbolizer in this way and regressed in
D122251 or thereabouts.

Trying this coupled with a reland of patch that failed on a couple of bots with
no useful stacktrace...

llvm/utils/lit/lit/TestingConfig.py

@@ -29,6 +29,7 @@ class TestingConfig(object):
             'CLANG',
             'LLDB',
             'LD_PRELOAD',
+            'LLVM_SYMBOLIZER_PATH',
             'ASAN_SYMBOLIZER_PATH',
             'LSAN_SYMBOLIZER_PATH',
             'MSAN_SYMBOLIZER_PATH',