tests: fix many spurious failures in VPATH builds

tests: fix many spurious failures in VPATH builds

The 'is' helper script, used by some test cases since commit
'v1.11-1830-g96401cb' of 08-02-2012, "tests: better way to compare
lists in Makefile rules", is not available in VPATH builds, since
it resides in $(srcdir), nor in $(builddir), and only $(builddir)
is added to $PATH in the test script.  This was causing spurious
failures.

Solve this issue by re-organizing a bit the layout and placing of
wrapper and help scripts used by the testsuite.  This reorganization
is by no mean complete, but should be enough to solve the VPATH
issues.

* tests/is: Moved ...
* tests/ax/is: ... here
* tests/automake.in: Moved ...
* tests/wrap/automake.in: ... here.
* tests/aclocal.in: Moved ...
* tests/wrap/aclocal.in: ... here.
* tests/defs-static.in ($am_bindir, $PATH): Adjust.
* configure.ac (AC_CONFIG_FILES): Likewise.
* tests/Makefile.am (EXTRA_DIST, check_SCRIPTS): Likewise.
* tests/get-sysconf.test: Likewise.
* .gitignore: Likewise.