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)