autotest: enable usage of EXEEXT in AT_TESTED

autotest: enable usage of EXEEXT in AT_TESTED

Together with Linux's binfmt-misc feature, Wine can be used to test
cross-compiled programs as if they were native.  However, the shell
will not perform the "magic" addition of the .exe extension after a
program name when searching for an executable.  These simple patches
let the user work around this by specifying $EXEEXT in the AT_CHECK
and AT_TESTED argument.  (More care is needed because of carriage
returns, but this is beyond the scope of this series).

* tests/autotest.at (AT_INIT): Expand contents of $at_tested.
(AT_TESTED): Quote each program that is passed to the function.
* lib/autotest/general.m4 (C unit tests): Add AT_TESTED invocation
and keyword.
* doc/autoconf.texi (Writing testsuites): Document usage of variables
in AT_TESTED.
* NEWS: Document change.

Signed-off-by: Paolo Bonzini <bonzini@gnu.org>