tests: automatic re-execution works for non-POSIX shells too

tests: automatic re-execution works for non-POSIX shells too

Some inferior shells are too greedy in parsing their input.  If a
non-POSIX Bourne shell (like Solaris 10 /bin/sh) was used to launch
one of our test scripts, it would fail unconditionally, because it
unexpectedly saw some (by it) unsupported constructs, notwithstanding
such constructs being placed *after* the code implementing automatic
test re-execution with a better shell.  In conclusion, the shell
bailed out like this:

    $ /bin/sh t/ar.sh
    $ t/ar.sh: syntax error at line 257: `is_newest_files=$' unexpected

By moving all the potentially problematic code in a separate file, to
be sourced only after the code for automatic re-execution with a better
shell, we ensure that inferior shell cannot see such code by mistake.

* defs: All code after automatic shell re-execution moved out ...
* t/ax/test-init.sh: ... to this new file.
* syntax-checks.mk (xdefs): Add it.
* Makefile.am (dist_check_DATA): Add it.  Also move in 'defs' from
a less explicit 'check_DATA' declaration.
(nodist_check_DATA):  Move in 'defs-static' from a less explicit
'check_DATA' declaration.
(check_DATA): Remove.
* t/self-check-sanity.sh: Remove, it was actually too hacky and brittle,
sanity-checking situations we don0t actually care about.
* t/list-of-tests.mk: Adjust.
* t/self-check-explicit-skips.sh: Adjust, and fix a botched heading
comments while we are at it.
* t/self-check-reexec.tap: Adjust.
* t/self-check-cleanup.tap: Likewise.

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