vala tests: source test-init.sh, not ./defs

vala tests: source test-init.sh, not ./defs

This will avoid spurious errors when the new vala tests (introduced
in the 'vala-work' branch) will be merged back to master.

* t/vala-headers.sh: Adjust as said.
* t/vala-libs.sh: Likewise.
* t/vala-mix.sh: Likewise.
* t/vala-mix2.sh: Likewise.
* t/vala-parallel.sh: Likewise.
* t/vala-vapi.sh: Likewise.
* t/vala-vpath.sh: Likewise.
* t/vala.sh: Likewise.
* t/vala2.sh: Likewise.
* t/vala3.sh: Likewise.
* t/vala4.sh: Likewise.
* t/vala5.sh: Likewise.

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