tests: improve testing (#80)

* chore: setup mini.test

* chore(ci): setup new test runner, install plugin deps

* chore(ci): test only on stable and nightly releases

* test: iferr

* test: struct_tags

* test: impl

* test: gotests

spec/testutils.lua

@@ -0,0 +1,43 @@
+local base_dir = vim.env.GOPHER_DIR or vim.fn.expand "%:p:h"
+
+---@class gopher.TestUtils
+local testutils = {}
+
+testutils.mininit_path = vim.fs.joinpath(base_dir, "scripts", "minimal_init.lua")
+testutils.fixtures_dir = vim.fs.joinpath(base_dir, "spec/fixtures")
+
+---@generic T
+---@param a T
+---@param b T
+---@return boolean
+function testutils.eq(a, b)
+  return MiniTest.expect.equality(a, b)
+end
+
+---@return string
+function testutils.tmpfile()
+  return vim.fn.tempname() .. ".go"
+end
+
+---@param path string
+---@return string
+function testutils.readfile(path)
+  return vim.fn.join(vim.fn.readfile(path), "\n")
+end
+
+---@param fpath string
+---@param contents string
+function testutils.writefile(fpath, contents)
+  vim.fn.writefile(vim.split(contents, "\n"), fpath)
+end
+
+---@param fixture string
+---@return {input: string, output: string}
+function testutils.get_fixtures(fixture)
+  return {
+    input = testutils.readfile(vim.fs.joinpath(testutils.fixtures_dir, fixture) .. "_input.go"),
+    output = testutils.readfile(vim.fs.joinpath(testutils.fixtures_dir, fixture) .. "_output.go"),
+  }
+end
+
+return testutils