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

scripts/meson.build

@@ -0,0 +1,14 @@
+configure_file(
+  input : 'nix-profile.sh.in',
+  output : 'nix-profile.sh',
+  configuration : {
+    'localstatedir': state_dir,
+  }
+)
+
+# https://github.com/mesonbuild/meson/issues/860
+configure_file(
+  input : 'nix-profile.sh.in',
+  output : 'nix-profile.sh.in',
+  copy : true,
+)