tests: minor reorganization of few tests

tests: minor reorganization of few tests

* t/subdir3.sh: Rename ...
* t/sourcefile-in-subdir.sh: ... like this.
* t/subdir4.sh: Rename ...
* t/depcomp-implicit-auxdir.sh: ... like this.
* t/subdir6.sh: Rename ...
* t/confh-subdir-clean.sh: ... like this.
* t/subdir7.sh: Renamed ...
* t/dir-named-obj-is-bad.sh: ... like this, and enhance.
* t/srcsub.sh, t/srcsub2.sh: Unify ...
* t/src-acsubst.sh: ... as this test.
* t/list-of-tests.mk: Adjust.

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