tests: don't postprocess stderr with sed
On Solaris, sed throws away the NUL bytes from the stream, even in C
locale. So instead of postprocessing bison's stderr to neutralize
changes in value of `argv[0]`, use an envvar to actually neutralize
variations of `argv[0]` during tests.
Reported by Dagobert Michelsen.
<https://lists.gnu.org/r/bug-bison/2021-08/msg00025.html>
* src/main.c (main): Change `argv[0]` if BISON_PROGRAM_NAME is
defined.
* tests/bison.in: No longer mess with stderr, just pass
the expected BISON_PROGRAM_NAME value.