tests: do not assume AM_MAKEINFOHTMLFLAGS is used in non-html targets

tests: do not assume AM_MAKEINFOHTMLFLAGS is used in non-html targets

Fixes automake bug https://bugs.gnu.org/30172.

Since AM_MAKEINFOHTMLFLAGS overrides AM_MAKEINFOFLAGS only for html
targets, make sure we restore the hacked up makefile before testing
the non-html formats.  This normally doesn't cause a problem for most
people, but if their tex install doesn't work correctly, texi2dvi will
fallback to running $MAKEINFO and using invalid flags in the process.

* THANKS: Add Mathieu Lirzin.
* t/txinfo-many-output-formats.sh: Restore Makefile.am after testing
html targets.
* t/txinfo-many-output-formats-vpath.sh: Likewise.