Set LLVM_SYMBOLIZER_PATH in test_env.py.

Hopefully it would fix mysterious crashes of browser_tests under ASAN
on Swarming mac. runtest.py (that is NOT used by Swarming) does set
LLVM_SYMBOLIZER_PATH already. Looks like this bit of logic wasn't copy-pasted
into test_env.py.

BUG=424280
R=earthdok@chromium.org,glider@chromium.org

Review URL: https://codereview.chromium.org/665873002

Cr-Commit-Position: refs/heads/master@{#300244}

testing/test_env.py

@@ -114,6 +114,8 @@ def get_asan_env(cmd, lsan):
     # Important note: when running ASan with leak detection enabled, we must use
     # the LSan symbolization options above.
     symbolization_options = ['symbolize=0']
+    # Set the path to llvm-symbolizer to be used by asan_symbolize.py
+    extra_env['LLVM_SYMBOLIZER_PATH'] = symbolizer_path
 
   asan_options.extend(symbolization_options)