Expose API to control rate and max prior attempts of collect_a_little

Expose API to control rate and max prior attempts of collect_a_little
(a cherry-pick of commit d897344 from 'unity-release-7_4-incremental')

* alloc.c (GC_RATE): Update comment.
* alloc.c (GC_rate): New STATIC variable (initialized to GC_RATE).
* alloc.c (GC_set_rate, GC_get_rate, GC_set_max_prior_attempts,
GC_get_max_prior_attempts): New API function definition.
* alloc.c (max_prior_attempts): New static variable (initialized to
MAX_PRIOR_ATTEMPTS).
* alloc.c (GC_collect_a_little_inner): Replace GC_RATE to GC_rate;
declare max_deficit local variable and compute GC_rate*n once (per
a function call); replace MAX_PRIOR_ATTEMPTS to max_prior_attempts.
* include/gc.h (GC_set_rate, GC_get_rate, GC_set_max_prior_attempts,
GC_get_max_prior_attempts): New API function declaration.
* tests/test.c [GC_PTHREADS] (main): Call GC_set_rate, GC_get_rate,
GC_set_max_prior_attempts, GC_get_max_prior_attempts.