runner: const-correct the unit tests

const where possible, and cast away const when passing argv to
parse_options, which expects non-const strings, because it passes them
to getopt_long, which expects non-const strings...

getopt and getopt_long take the argv array as char * const *, or in
other words, as pointer-to-const-pointer-to-char. In C, pointer-to-T
implicitly converts to pointer-to-const-T and for a char **, the T is
char* and "const T" is char * const, ergo char ** converts to char *
const *, not const char **. The only const-correctness getopt and
getopt_long can really do is char * const * or they lose the ability
to directly pass in main()'s arguments, which are an array of
non-const pointers to non-const char for legacy reasons.

For testing the argument handling, it's very convenient to use an
array of string literals, which are of type const char[N], convertible
to const char *. To get such an array into getopt, the choices are:

1) Cast away the const in the pointer-to-pointer
2) Cast away the const in the string literal
3) Don't cast anything and eat the compiler warning

Option 1 looked cleanest out of all those.

tl;dr: Choices made in 1972 force our hand.

v2:
 - Augment commit message

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>

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