tests: give few vala tests more significant names

tests: give few vala tests more significant names

* t/vala.sh: Rename ...
* t/vala-grepping.sh: ... like this.
* t/vala2.sh: Rename ...
* t/vala-recursive-setup.sh: ... like this.
* t/vala3.sh: Rename ...
* t/vala-non-recursive-setup.sh: ... like this.
* t/vala4.sh: Rename ...
* t/vala-configure.sh: ... like this.
* t/vala5.sh: Rename ...
* t/vala-per-target-flags.sh: ... like this, and slightly
improve heading comments.
* t/list-of-tests.mk: Adjust.

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