tests: Implement a way to test nixvim (#199)

The tests can be executed using `nix flake check`, they check that
modules can be built, and they execute in neovim without any errors.

This commit only implements tests for tokyonight-nvim upstream defaults

tests/plugins/default.nix

@@ -0,0 +1,23 @@
+{lib}: let
+  # List of files containing configurations
+  pluginFiles =
+    builtins.filter (p: p != "default.nix") (builtins.attrNames (builtins.readDir ./.));
+
+  /*
+  Create a list of tests. The list is of the form:
+    [ { name = "<plugin>-<test_name>"; value = { ... }; } ]
+  */
+  makePluginTests = pluginFile: let
+    pluginName = builtins.head (lib.strings.splitString "." pluginFile);
+    pluginConfigs = import (./. + "/${pluginFile}");
+  in
+    lib.attrsets.mapAttrsToList (testName: testConfig: {
+      name = "${pluginName}-${testName}";
+      value = testConfig;
+    })
+    pluginConfigs;
+
+  # A list of lists of test cases for each plugin
+  pluginTests = builtins.map makePluginTests pluginFiles;
+in
+  builtins.listToAttrs (lib.lists.flatten pluginTests)

tests/plugins/tokyonight.nix

@@ -0,0 +1,31 @@
+{
+  # All the upstream default options of tokyonight
+  defaults = {
+    colorschemes.tokyonight = {
+      enable = true;
+
+      style = "storm";
+      # Not implemented
+      # lightStyle = "day";
+      transparent = false;
+      terminalColors = true;
+      styles = {
+        comments = {italic = true;};
+        keywords = {italic = true;};
+        functions = {};
+        variables = {};
+        sidebars = "dark";
+        floats = "dark";
+      };
+      sidebars = ["qf" "help"];
+      dayBrightness = 0.3;
+      hideInactiveStatusline = false;
+      dimInactive = false;
+      lualineBold = false;
+      # Not implemented
+      # onColors = {__raw = "function(colors) end";};
+      # Not implemented
+      # onHighlights = {__raw = "function(colors) end";};
+    };
+  };
+}