tests: use append mode to capture parallel make output

tests: use append mode to capture parallel make output

This should fix the spurious failure reported in automake bug#11413.

This is due to the fact that the redirected output of parallel make
can racily loose lines.  For example, if GNU make (3.82) is run on
a Makefile like this:

  all = 0 1 2 3 4 5 6 7 8 9
  default: $(all)
  $(all):
        @sleep 0.$$(($RANDOM % 10)); echo $@

and has its standard output redirected in overwrite-mode to a regular
file, it looses a line of that output every 15 runs or so on a Fedora
17 system with 64 ppc64 cores and Linux 3.3.1.  Redirection in append
mode does not suffer of this problem.

See also similar commit 'Release-1-10-280-g6426999' of 2009-03-10,
"Use append mode to capture parallel test output", which tackled a
similar problem for 't/parallel-tests3.sh' and 't/lisp8.sh'.

* t/tap-more.sh: Use append mode for output from "make -j", to avoid
dropped lines.
* t/parallel-tests3.sh, t/lisp8.sh: Enhance comments.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>