doc: test-driver option args are separate words.

doc: test-driver option args are separate words.

For https://bugs.gnu.org/70638.

As now mentioned in the manual, the description of the
test-driver command-line interface has been wrong since 2011
when it was first added. The manual, and the driver usage
messages themselves, have consistently shown option arguments
joined to the option name with `=' characters, while the
implementation of Automake has always provided option arguments
in the following argument word, and the provided test drivers
only accepted option arguments in the following argument word.

Because Automake has never used the `=' syntax, there can't
possibly be a working driver which actually follows the
specification as written, so, despite the fierce language of the
manual, we agreed that changing the specification was the right
approach. The `test-driver' program's usage message has already
been fixed (#22445), but the full extent of the problem wasn't
noticed at that time.

* doc/automake.texi (Command-line arguments for test drivers): Fix the
table of options to show arguments passed as separate words; add
footnote explaining this rather sorry situation.
* doc/automake.texi (Use TAP with the Automake test harness):
Remove `=' from documentation of `--diagnostic-string', because
that was never acceptable either.
* lib/tap-driver.sh: Fix usage message.
* contrib/tap-driver.pl: Change usage message to match the defined
protocol.  (This implementation parses options using Perl's
`Getopt::Long' module, so it accepts the `=' syntax as specified, but
this program isn't actually used.)