feat: decide whether two C strings are equal and support CHECK_STREQ* and CHECK_STRNE*

[empiredan]

This PR is to resolve #1299.

There are several types of comparisons for equality between two strings:

• if both strings are of std::string, to compare case-sensitive equality, we can use operator==
• if both strings are of std::string, to compare case-insensitive equality, we can use boost::iequals
• if one string is of std::string and another is of C string, to compare case-sensitive equality we can also use operator==
• if one string is of std::string and another is of C string, to compare case-insensitive equality we can use strcasecmp
• if both strings are of C string, to compare case-sensitive equality we can use strcmp
• if both strings are of C string, to compare case-insensitive equality we can also use strcasecmp
• to compare the equality of first n bytes, we can use strncmp, strncasecmp, memcmp

However, the problem of strcmp, strcmp, strcasecmp strncmp, strncasecmp and memcmp is that once the compared strings is NULL pointer, these functions may lead to undefined behavior. Thus in this PR we will encapsulate the following functions to solve this problem:

// Decide whether two C strings are equal, even if one of them is NULL.
// The second function is similar except it compares the only first (at most) n bytes
// of both strings.
bool equals(const char *lhs, const char *rhs);
bool equals(const char *lhs, const char *rhs, size_t n);

// Decide whether two C strings are equal, ignoring the case of the characters,
// even if one of them is NULL.
// The second function is similar except it compares the only first (at most) n bytes
// of both strings.
bool iequals(const char *lhs, const char *rhs);
bool iequals(const char *lhs, const char *rhs, size_t n);

// Decide whether two strings one of which is C string are equal, ignoring the
// case of the characters, even if the C string is NULL.
bool iequals(const std::string &lhs, const char *rhs);
bool iequals(const std::string &lhs, const char *rhs, size_t n);
bool iequals(const char *lhs, const std::string &rhs);
bool iequals(const char *lhs, const std::string &rhs, size_t n);

// Decide whether the first n bytes of two memory areas are equal, even if one of them is NULL.
bool mequals(const void *lhs, const void *rhs, size_t n);

This PR will also implement the following macros to help check the equality between C strings and true or false:

• CHECK_STREQ*
• CHECK_STRNE*
• CHECK_TRUE
• CHECK_FALSE

[acelyc111]

How about to replace memcmp too?

[empiredan]

How about to replace memcmp too?

OK, I think I can replace with a new function mequals that also processes NULL pointers.

[acelyc111]

how about define the parameter as string_view? so you it's not needed to convet to C string.

[empiredan]

Actually string_view is used not only for the string of the printable characters, but also binary bytes. Once string_view is used as the type of the parameters, we have to reimplement comparison ignoring cases based on string_view where binary bytes such as '\0' should be skipped. Therefore here we just declare other functions one parameter of which is declared as std::string. Comparing two std::string-typed strings we can use boost::iequals.

[acelyc111]

This style of unit test is wildly used, but it has a shortcoming, when some case failed, it's hard to find out which one failed, right?

[empiredan]

Yes, once a case has failed it is really hard to look out it. I'll use TEST_P to refactor this.