[BACKEND] Add C++ tests for PTXFormat and some tiny refinement (#109)

This PR does

1. Add some C++ tests for `PTXFormat`
2. Enhance the functionality of `PTXFormat`, make a `PTXInstr` instance
can be called multiple times similar as a C function.

unittest/CMakeLists.txt

@@ -16,6 +16,7 @@ function(add_triton_ut)
           ${__SRCS})
   target_link_libraries(
           ${__NAME}
+          PRIVATE
           GTest::gtest_main
           gmock
           ${__LIBS})