Fix unit test UB resulting from incorrect initialization

[kingcrimsontianyu]

KvikIO unit test has a utility class EnvVarContext class, introduced in #700, and slightly improved in #735. It has been identified that this class was incorrectly initialized, resulting in UB: it causes the unit test failure in C++20, which by fluke was not observed in C++17. This PR fixes this error. Specifically, the constructor of EnvVarContext is:

EnvVarContext(std::initializer_list<std::pair<std::string_view, std::string_view>> env_var_entries);

There are several ways of instantiation:

// Direct initialization
EnvVarContext env_var_ctx({{"env_1", "v1"}, {"env_2", "v2"}});

// Direct list initialization
EnvVarContext env_var_ctx{{"env_1", "v1"}, {"env_2", "v2"}};

// Copy list initialization
EnvVarContext env_var_ctx = {{"env_1", "v1"}, {"env_2", "v2"}};

The erroneous instantiation performed is:

// Extra pair of braces
// {}: brace-enclosed initializer list
// {{"env_1", "v1"}, {"env_2", "v2"}}: one element of type pair<std::string_view, std::string_view>
// {"env_1", "v1"}: first
// {"env_2", "v2"}: second
EnvVarContext env_var_ctx{{{"env_1", "v1"}, {"env_2", "v2"}}};

As a result, the initializer list only has 1 pair, with the key being {"env_1", "v1"} and value being {"env_2", "v2"}. For the key, for instance, the 5-th overload (https://en.cppreference.com/w/cpp/string/basic_string_view/basic_string_view.html) of the constructor was used, where first points to "env_1" and last points to "v1". Since the two iterators do not form a valid range, UB ensues.

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[madsbk]

Thanks @kingcrimsontianyu

[vuule]

/merge