unittests: Add a test for TV_CSTRING

Not using enum{} because SIZE_MAX exceeds integer and I do not really like how
enum definition is described in C99:

1. Even though all values must fit into the chosen type (6.7.2.2, p 4) the type
   to choose is still implementation-defined.
2. 6.4.4.3 explicitly states that “an identifier declared as an enumeration
   constant has type `int`”. So it looks like “no matter what type was chosen
   for enumeration, constants will be integers”. Yet the following simple
   program:

        #include <stdint.h>
        #include <stdio.h>
        #include <stddef.h>

        enum { X=SIZE_MAX };

        int main(int argc, char **argv)
        {
          printf("x:%zu m:%zu t:%zu v:%zu",
                 sizeof(X), sizeof(SIZE_MAX), sizeof(size_t), (size_t)X);
        }

    yields one of the following using different compilers:

    - clang/gcc/pathcc: `x:8 m:8 t:8 v:18446744073709551615`
    - pcc/tcc: `x:4 m:8 t:8 v:1844674407370955161`

    If I remove the cast of X to size_t then pcc/tcc both yield `x:4 m:8 t:8
    v:4294967295`, other compilers’ output does not change.

    All compilers were called with `$compiler -std=c99 -xc -` (feeding program
    from echo), except for `tcc` which has missing `-std=c99`. `pcc` seems to
    ignore the argument though: it is perfectly fine with `-std=c1000`.

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