- fix false test failure on picolibc
- For picolibc, the result of mallinfo().uordblks doesn't go back
to zero after the first allocation. By adding `free(malloc(4))`
before recording the pre-test mallinfo() value, this issues is
worked arround
- increase test coverage to also cover realloc()
- ease testing with picolibc by allowing `make PICOLIBC=1 flash test`