tests: prefer "test ! -e FILE" to check that a file doesn't exist

tests: prefer "test ! -e FILE" to check that a file doesn't exist

Once, for the sake of (at least) Solaris 10 /bin/sh, we had to use
"test ! -f FILE" or "test ! -r FILE" or "test ! -d FILE" instead,
because the that shell's 'test' built-in didn't grok the '-e' option.

Note however that we still can't use "test ! -e" in the Makefile recipes
used in the test cases; that is because those recipes are run with the
shell detected by 'configure', and Autoconf-generated configure scripts
do no guarantee to find or provide a POSIX-compatible shell.

* Several tests: Adjust.
* t/yacc-clean-cxx: Adjust, and remove a couple of useless commands.
* t/parallel-tests-dry-run-2.sh: Adjust, and add invocation to
"make -n" forgotten in previous versions of the test.
* t/txinfo26.sh: Adjust, and don't bother to skip the test when it's
run in a directory whose absolute path contain whitespace: that setup
is not supported anyway.
* t/maken3.sh: Adjust, and fix a typo that could cause a minor false
negative.
* t/test-trs-recover2.sh: Prefer using 'skip_' with a suitable error
message over a bare 'exit 77'.

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