Tests: introduced a separate cache directory for Go builds.

The Go compiler can't detect changes to C header files when
compiling CGO applications, and then this leads to Go test
samples being linked with wrong libunit.

This patch creates a new cache directory reused throughout
the test suite.

test/unit/applications/lang/go.py

@@ -12,6 +12,7 @@ class TestApplicationGo(TestApplicationProto):
 
         env = os.environ.copy()
         env['GOPATH'] = option.current_dir + '/build/go'
+        env['GOCACHE'] = option.cache_dir + '/go'
 
         if static:
             args = [