fix: Missing comma in api_build_test.py

* The comma has been most probably left out unintentionally, leading to string concatenation between the two consecutive lines. It has been found automatically using a regular expression.

tests/ssh/api_build_test.py

@@ -253,7 +253,7 @@ def test_build_container_with_target(self):
             'RUN mkdir -p /tmp/test',
             'RUN touch /tmp/silence.tar.gz',
             'FROM alpine:latest',
-            'WORKDIR /root/'
+            'WORKDIR /root/',
             'COPY --from=first /tmp/silence.tar.gz .',
             'ONBUILD RUN echo "This should not be in the final image"'
         ]).encode('ascii'))