Disable ConcurrencyCheck by default and set using environment variable

Currently, it is disabled at compile time. Will be quite useful to be able
turn this on in system test setup, especially in large scale setups

CONCURRENCY_CHECK_ENABLE is set in scons before launching any unit test

Change-Id: I8ca117a282c5556a70859c2acf284e9ef5f92a49
Closes-Bug: 1681907

rules.py

@@ -111,6 +111,8 @@ def RunUnitTest(env, target, source, timeout = 300):
         # https://code.google.com/p/gperftools/issues/detail?id=497
         ShEnv['LD_BIND_NOW'] = '1'
 
+    if 'CONCURRENCY_CHECK_ENABLE' not in ShEnv:
+        ShEnv['CONCURRENCY_CHECK_ENABLE'] = 'true'
     proc = subprocess.Popen(cmd, stdout=logfile, stderr=logfile, env=ShEnv)
 
     # 60 second timeout