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.

src/main.c

@@ -63,6 +63,12 @@
 int
 main (int argc, char *argv[])
 {
+  {
+    char *cp = getenv ("BISON_PROGRAM_NAME");
+    if (cp)
+      argv[0] = cp;
+  }
+
 #define DEPENDS_ON_LIBINTL 1
   set_program_name (argv[0]);
   setlocale (LC_ALL, "");