Modernize and improve test scripts `subdir*.test'.

Modernize and improve test scripts `subdir*.test'.

* tests/subdir.test: Enable `errexit' shell flag, and related
changes.  Use the `configure.in' stub created by `./defs',
rather than writing one from scratch.
* tests/subdir2.test: Likewise.
* tests/subdir4.test: Likewise.
* tests/subdir3.test: Enable `errexit' shell flag, and related
changes.  Do not create useless dummy source files.
* tests/subdir5.test: Make grepping of Makefile.in (in topdir
and in subdirs) stricter.  Some minor changes to keep it more
in sync with the related test `subdir8.test'.
* tests/subdir8.test: Likewise (but with the related test being
`subdir5.test' here).
* tests/subdir6.test: Cosmetic change in spacing.
* tests/subdir9.test: Define and use new variable `$distdir'.  Add
trailing `:' command.
* tests/subdir10.test: Cosmetic consistency-related change.
* tests/subdirbuiltsources.test: Cosmetic changes in spacings.
(configure.in): Use stub created by `./defs', rather than writing
it from scratch.  Do not use obsoleted and/or deprecated forms of
autoconf/automake macros.