testsuite: yet more use of TAP, and related extensions

testsuite: yet more use of TAP, and related extensions

* tests/defs (show_): New function, display the contents of one or
more files on stdout, with bells & whistles (both for cosmetic and
practical reasons, the latter aimed at avoiding spurious TAP
diagnostic).
(AUTOMAKE_run): Extend and adapt to make it compatible with TAP
based tests.  Since we are at it, make its implementation more
namespace-safe, and improve its command-line interface.
(AUTOMAKE_fails): Adapt to the new command-line interface of
`AUTOMAKE_run'.
* tests/add-missing.test: Renamed ...
* tests/add-missing.tap: ... to this, and converted to the use
of the TAP protocol, and of the new `show_' function.
* tests/automake.test: Renamed ...
* tests/automake-cmdline.tap: ... to this, and converted to the
use of the TAP protocol.
* tests/getopt.test:  Adapt to the new command-line interface
of `AUTOMAKE_run'.
* tests/cond44.test: Likewise.
* tests/cond45.test: Likewise.
* tests/configure.test: Likewise.
* tests/license2.test: Likewise.
* tests/parallel-am.test: Likewise.
* tests/parallel-am3.test: Likewise.
* tests/tags2.test: Likewise.
* tests/werror3.test: Likewise.
* tests/werror4.test: Likewise.
* tests/amopts-variable-expansion.test: Likewise.
* tests/warnings-win-over-strictness.test: Likewise.
* tests/Makefile.am (TESTS): Update.
(TAP_LOG_DRIVER_FLAGS): Add `--merge', so that TAP diagnostic is
reported in the testsuite progress output.