tests: move coverage about BUILT_SOURCES
Basically a backport of some tests from Automake-NG.
* t/built-sources-check.sh: Sync it with the version in the ng/master
branch. Accordingly, move part of the checks out ...
* t/built-sources-install.sh: ... into this new test, synced from
ng/master as well.
* t/built-sources-subdir.sh: Minor tweaks and enhancements to sync it
with the version in ng/master.
* t/built-sources-cond.sh: New test, synced from ng/master.
* t/built-sources.sh: Likewise, with minor edits to avoid a spurious
failure.
* t/built-sources-fork-bomb.sh: Likewise.
* t/list-of-tests.mk: Update.
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>