Add indent tests

I made use of the 'nvim-treesitter' test runner for these.

test/indent/groupings.tig

@@ -0,0 +1,10 @@
+let
+  var a := 42
+in
+  (
+    12;
+    27;
+    42
+  );
+  a
+end

test/indent_spec.lua

@@ -0,0 +1,39 @@
+package.path = package.path .. ";" .. vim.env.NVIM_TREESITTER .. "/?.lua"
+
+local Runner = require("tests.indent.common").Runner
+local XFAIL = require("tests.indent.common").XFAIL
+
+local runner = Runner:new(it, "./test/indent/", {
+    tabstop = 2,
+    shiftwidth = 2,
+    softtabstop = 0,
+    expandtab = true,
+})
+
+describe("indent Tiger:", function()
+    describe("whole file:", function()
+        runner:whole_file("../highlight/", {
+            expected_failures = {
+                -- NOTE: none for now
+            },
+        })
+        runner:whole_file("../tags/", {
+            expected_failures = {
+                -- NOTE: none for now
+            },
+        })
+        runner:whole_file(".", {
+            expected_failures = {
+                -- NOTE: none for now
+            },
+        })
+    end)
+
+    describe("new line:", function()
+        runner:new_line("groupings.tig", { on_line = 2, text = "var b := 0", indent = 2 }, "let declarations")
+        runner:new_line("groupings.tig", { on_line = 3, text = "a := a + 1", indent = 2 }, "after 'in'", XFAIL)
+        runner:new_line("groupings.tig", { on_line = 4, text = "a := a + 1;", indent = 4 }, "sequence", XFAIL)
+        runner:new_line("groupings.tig", { on_line = 8, text = "a := a + 1;", indent = 2 }, "after sequence")
+        runner:new_line("groupings.tig", { on_line = 10, text = "+ 1", indent = 0 }, "after 'end'")
+    end)
+end)