Go: fixing tests for Go 1.16.

In Go 1.16, the module-aware mode is enabled by default; to fall back to
previous behavior, the GO111MODULE environment variable should be set to
'auto'.

Details: https://golang.org/doc/go1.16

test/unit/check/go.py

@@ -8,6 +8,7 @@ def check_go(current_dir, temp_dir, test_dir):
 
     env = os.environ.copy()
     env['GOPATH'] = current_dir + '/build/go'
+    env['GO111MODULE'] = 'auto'
 
     try:
         process = subprocess.Popen(