tests: few minor fixlets, improvements and tweakings

tests: few minor fixlets, improvements and tweakings

* tests/defs-static.in ($top_testbuilddir): New, for consistency
and completeness.
* tests/Makefile.am (do_subst): Substitute @abs_top_testbuilddir@
as well.
* tests/java-compile-run-mested.test: Prefer AM_TESTS_ENVIRONMENT
over TESTS_ENVIRONMENT, now that the former has become available.
* tests/java-compile-run-flat.test: Correct a botched "FIXME"
comment.
* tests/tap-realtime.test: Remove extra whitespace in comments.
* tests/missing-tar.test: Use `get_shell_script' to bring in the
`missing' script, to increase coverage.  Remove redundant call to
`set -e'.  Fix a typo in comments.
* tests/get-sysconf.test: Remove redundant definitions of
`$top_testsrcdir', `testbuilddir' and `$top_testbuilddir'.