argv is missing nullptr at the end in TestParallelHarness

[cz4rs]

Describe the bug
For strict standards compliance (i.e. if the parser in cli11 relies on it now or in the future), nullptr should be pushed on the end of a vector storing argv.

• https://stackoverflow.com/questions/16418932/is-argvargc-equal-to-null-pointer

TestParallelHarness uses additional_args_ to store arguments, which lacks nullptr at argv[argc].

vt/tests/unit/test_parallel_harness.h

Lines 152 to 184 in 3f7d761

	std::pair<int, char**>
	injectAdditionalArgs(int old_argc, char** old_argv) {
	std::copy(
	old_argv, old_argv + old_argc, std::back_inserter(additional_args_)
	);
	addAdditionalArgs();
	int custom_argc = additional_args_.size();
	char** custom_argv = additional_args_.data();
	return std::make_pair(custom_argc, custom_argv);
	}
	/**
	* \internal \brief Add additional arguments used during initialization of vt
	* components
	*
	* To add additional arguments override this function in your class and add
	* needed arguments to `additional_args_` vector.
	*
	* Example:
	* struct TestParallelHarnessWithStatsDumping : TestParallelHarnessParam<int> {
	* virtual void addAdditionalArgs() override {
	* static char vt_lb_stats[]{"--vt_lb_stats"};
	* static char vt_lb_stats_dir[]{"--vt_lb_stats_dir=test_stats_dir"};
	* static char vt_lb_stats_file[]{"--vt_lb_stats_file=test_stats_outfile"};
	*
	* addArgs(vt_lb_stats, vt_lb_stats_dir, vt_lb_stats_file);
	* }
	* };
	*/
	virtual void addAdditionalArgs() {}

Expected behavior
Add nullptr as required by the standard. See 1ce0310 for reference.

[cz4rs]

note: TestHarness seems to be missing it as well

vt/tests/unit/test_harness.h

Lines 62 to 83 in 43f4f21

	virtual void SetUp() {
	argc_ = orig_args_.size();
	argv_ = new char*[argc_];
	for (int i = 0; i < argc_; i++) {
	auto const len = orig_args_[i].size();
	argv_[i] = new char[len + 1];
	argv_[i][len] = 0;
	orig_args_[i].copy(argv_[i], len);
	}
	}
	virtual void TearDown() {
	for (int i = 0; i < argc_; i++) {
	delete [] argv_[i];
	}
	delete [] argv_;
	}
	static void store_cmdline_args(int argc, char **argv) {
	orig_args_ = std::vector<std::string>(argv, argv + argc);
	}

edit: vt::tests::unit::TestHarnessAny<TestBase>::argv_ and vt::tests::unit::TestHarnessAny<TestBase>::argc_ are not used anywhere else and can probably be safely deleted (along with SetUp and TearDown methods)