meson: implement functional tests

Functional tests can be run with
`meson test -C build --suite installcheck`.

Notably, functional tests must be run *after* running `meson install`
(Lix's derivation runs the installcheck suite in installCheckPhase so it
does this correctly), due to some quirks between Meson and the testing
system.

As far as I can tell the functional tests are meant to be run after
installing anyway, but unfortunately I can't transparently make
`meson test --suite installcheck` depend on the install targets.

The script that runs the functional tests, meson/run-test.py, checks
that `meson install` has happened and fails fast with a (hopefully)
helpful error message if any of the functional tests are run before
installing.

TODO: this change needs reflection in developer documentation

Change-Id: I8dcb5fdfc0b6cb17580973d24ad930abd57018f6

src/meson.build

@@ -63,3 +63,7 @@ nix2_commands_sources = [
 # Finally, the nix command itself, which all of the other commands are implmented in terms of
 # as a multicall binary.
 subdir('nix')
+
+# Just copies nix-channel/unpack-channel.nix to the build directory.
+# Done as a subdir to get Meson to respect the path hierarchy.
+subdir('nix-channel')

src/nix-channel/meson.build

@@ -0,0 +1,5 @@
+configure_file(
+  input : 'unpack-channel.nix',
+  output : 'unpack-channel.nix',
+  copy : true,
+)