[CDRIVER-6107] Add Test Case Tag Support

[vector-of-bool]

Refer: CDRIVER-6107

This changeset introduces the ability to apply label/tags to test cases, and exposes those as CTest labels.

This also includes new general mutable string and array/vector handling utilities, and refactors a lot of our test suite to use those.

Changes:

• 6d908aa supports a because clause on all mlib_check assertions. This message is included in the output when the program terminates, and acts as inline documentation for why a particular assertion is present.
• d636e33 Brings mstr, the data-owning counterpart to mstr_view. This piece has been in a stash for a while, and was brought out as a better way to handle mutable strings.
• 9675ae5 Adds vec.t.h, a generic template header that is used to define vector types. Basically:
  • #define a type T, and then #include <mlib/vec.t.h>, and it will define a T_vec type that acts as a dynamically sized contiguous array of T objects.
  • Additional macros can be defined to add copy/destroy semantics to the vector objects.
• f97883e Adds mstr_trim for removing whitespace from string views.
• 47629d7 Refactors a lot of our test suite code to use mtsr and vec.t.h types. Also adds mlib/str_vec.h which just defines the mstr_vec for the very common "array of strings" type.
• b39b795 Adds support for specifying tags/labels associated with test cases:
  • The tags are specified within the same string as the test name. (There was an attempt to add a second string parameter for tags, but this was extremely cumbersome to update all of the test cases across the codebase to pass a second empty string, especially with all the different signatures for adding test cases. A future refactor may want to consolidate our many "TestSuite_Add" functions/macros.)
  • After the test case name, place one or more spaces, and then a list of [bracketed][tags].
  • To support exporting of tags for LoadTests.cmake, the --tests-cmake switch will print CMake code that defines all the test cases. I wanted to use JSON to emit the test cases, but CMake's JSON handling functionality is incredibly slow for very large JSON blobs, so directly emitting CMake code was chosen instead.
  • LoadTests.cmake will evaluate the emitted CMake code and use it to call add_test for all the test cases, as well as apply the test case labels and fixtures.
  • If a test has a tag [uses:foo], then LoadTests.cmake will add a FIXTURES_REQUIRED of mongoc/fixtures/foo. This currently only applies to the IMDS tests, but will eventually be used for other test case fixtures.