build: Avoid the doctemplates hack

build: Avoid the doctemplates hack

The hack that copies the doctemplates directory into the build
directory has stopped working with newer versions of Meson; while it's
possible to copy files, custom_target() cannot depend on a directory.
Additionally, the dependency has always been broken.

Instead, we enumerate the template files—after all, it's not like they
change a lot—and then we list them as dependencies for the test targets.

Fixes: #414
Change-Id: I59d3522851b91e3caaeda17bca882e3746d445c0
Signed-off-by: JinWang An <jinwang.an@samsung.com>