tests: avoid new failures due to nonexistent makeinfo.

[Ralf Wildenhues]

http://hydra.nixos.org/build/858836/log/raw and other logs show new
testsuite failures of txinfo.test and txinfo8.test:

| if /bin/sh 
/tmp/nix-build-nc6n3czfw0njkp2962fmqhcyj4p20fmp-automake-1.11a.drv-0/automake-1.11a/tests/txinfo.dir/missing
 --run makeinfo   -I . \
|  -o textutils.info textutils.texi; \
| then \
|   rc=0; \
|   CDPATH="${ZSH_VERSION+.}:" && cd .; \
| else \
|   rc=$?; \
|   CDPATH="${ZSH_VERSION+.}:" && cd . && \
|   $restore $backupdir/* `echo "./textutils.info" | sed 's|[^/]*$||'`; \
| fi; \
| rm -rf $backupdir; exit $rc
| 
/tmp/nix-build-nc6n3czfw0njkp2962fmqhcyj4p20fmp-automake-1.11a.drv-0/automake-1.11a/tests/txinfo.dir/missing:
 line 52: makeinfo: command not found
| WARNING: `makeinfo' is missing on your system.  You should only need it if
|          you modified a `.texi' or `.texinfo' file, or any other file
|          indirectly affecting the aspect of the manual.  The spurious
|          call might also be the consequence of using a buggy `make' (AIX,
|          DU, IRIX).  You might want to install the `Texinfo' package or
|          the `GNU make' package.  Grab either from any GNU archive site.
| make[1]: *** [textutils.info] Error 1
| make[1]: Leaving directory 
`/tmp/nix-build-nc6n3czfw0njkp2962fmqhcyj4p20fmp-automake-1.11a.drv-0/automake-1.11a/tests/txinfo.dir'
| make: *** [distdir] Error 2
| + exit_status=2

I'm pushing the fix below to tests-texinfo and merging to maint.

Cheers,
Ralf

    tests: avoid new failures due to nonexistent makeinfo.
    
    * tests/txinfo.test, tests/txinfo8.test: Create stub info files
    so 'missing --run makeinfo' won't fail even if makeinfo is not
    present.

diff --git a/tests/txinfo.test b/tests/txinfo.test
index 2e94486..4858f41 100755
--- a/tests/txinfo.test
+++ b/tests/txinfo.test
@@ -39,6 +39,8 @@ END
 
 echo '@setfilename textutils.info' > textutils.texi
 : > texinfo.tex
+# Avoid requiring makeinfo.
+: > textutils.info
 
 $ACLOCAL
 $AUTOCONF
diff --git a/tests/txinfo8.test b/tests/txinfo8.test
index 8dd24a4..6c57a40 100755
--- a/tests/txinfo8.test
+++ b/tests/txinfo8.test
@@ -43,6 +43,8 @@ test2: distdir
 END
 
 echo '@setfilename textutils.info' > textutils.texi
+# Avoid requiring makeinfo.
+: > textutils.info
 
 mkdir auxdir