Fix pathing issue when wat2wasm is not globally installed

Makefile

@@ -19,7 +19,7 @@ server:
 	python3.8 -m http.server
 
 test: venv/.done
-	venv/bin/pytest tests $(TEST_FLAGS)
+	WAT2WASM=$(WAT2WASM) venv/bin/pytest tests $(TEST_FLAGS)
 
 lint: venv/.done
 	venv/bin/pylint py2wasm

tests/integration/helpers.py

@@ -1,6 +1,8 @@
 import io
+import os
 import subprocess
 import sys
+
 from tempfile import NamedTemporaryFile
 
 from pywasm import binary
@@ -9,14 +11,16 @@ from pywasm import Runtime
 from py2wasm.utils import process
 
 def wat2wasm(code_wat):
+    path = os.environ.get('WAT2WASM', 'wat2wasm')
+
     with NamedTemporaryFile('w+t') as input_fp:
         input_fp.write(code_wat)
         input_fp.flush()
 
         with NamedTemporaryFile('w+b') as output_fp:
-            result = subprocess.run(
+            subprocess.run(
                 [
-                    'wat2wasm',
+                    path,
                     input_fp.name,
                     '-o',
                     output_fp.name,