Expose API to control the minimum bytes allocated before a GC occurs

Expose API to control the minimum bytes allocated before a GC occurs
(a cherry-pick of commit 4c0e58d from 'unity-release-7_4-incremental')

* alloc.c (min_bytes_allocd_minimum): New static variable.
* alloc.c (GC_set_min_bytes_allocd, GC_get_min_bytes_allocd): New API
function definition (to set/get min_bytes_allocd_minimum).
* alloc.c (min_bytes_allocd): Return min_bytes_allocd_minimum if
result is less than min_bytes_allocd_minimum.
* include/gc.h (GC_set_min_bytes_allocd, GC_get_min_bytes_allocd): New
API function declaration.
* tests/test.c [GC_PTHREADS] (main): Call GC_set_min_bytes_allocd()
and GC_get_min_bytes_allocd().