Update ci/test.xsh

Author: certik

ci/test.xsh

@@ -22,8 +22,8 @@ else:
     src/bin/lpython examples/expr2.py
     src/bin/lpython --backend=c examples/expr2.py
     cd integration_tests
-    python run_tests.py -j16 -b llvm cpython wasm
-    python run_tests.py -j16 -b llvm cpython wasm -f
+    python run_tests.py -j16 -b llvm cpython c wasm
+    python run_tests.py -j16 -b llvm cpython c wasm -f
 
     if $(uname).strip() == "Linux":
         python run_tests.py -j16 -b x86 wasm_x86 wasm_x64