Check $NODE environment variable when looking for node (#1792)

This allows me to run tests on a system where the default
installed node is not recent enough.

Author: sbc100

scripts/fuzz_relooper.py

@@ -372,7 +372,8 @@ def get_phi(j):
                               stdout=subprocess.PIPE,
                               stderr=subprocess.PIPE).communicate()[0]
   print '-'
-  slow_out = subprocess.Popen(['nodejs', 'fuzz.slow.js'],
+  node = os.getenv('NODE', 'nodejs')
+  slow_out = subprocess.Popen([node, 'fuzz.slow.js'],
                               stdout=subprocess.PIPE,
                               stderr=subprocess.PIPE).communicate()[0]
   print '_'

scripts/test/shared.py

@@ -157,7 +157,7 @@ def is_exe(fpath):
             which('gcc') or which('clang'))
 NATIVEXX = (os.environ.get('CXX') or which('mingw32-g++') or
             which('g++') or which('clang++'))
-NODEJS = which('nodejs') or which('node')
+NODEJS = os.getenv('NODE', which('nodejs') or which('node'))
 MOZJS = which('mozjs') or which('spidermonkey')
 EMCC = which('emcc')