autotest: keywords do not accumulate

[Ralf Wildenhues]

accumulation of keywords is broken (CVS HEAD as well as AUTOCONF-2_59):

| $ tests/testsuite -k autoupdate -k autoreconf --list
| GNU Autoconf 2.59c test suite test groups:
| 
|  NUM: FILE-NAME:LINE     TEST-GROUP-NAME
|       KEYWORDS
| 
| $ tests/testsuite -k autoupdate -k autoreconf
| ## ------------------------------ ##
| ## GNU Autoconf 2.59c test suite. ##
| ## ------------------------------ ##
| 
| ## ------------- ##
| ## Test results. ##
| ## ------------- ##
| 
| 0 tests were successful.
| $

I remember to have seen this working at some point not too long ago.
OTOH, since this is broken in 2.59 as well, I didn't know where to start
a search for the bad patch.  Tested on x86_64-unknown-linux-gnu.

Regards,
Ralf