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/fixtures/impl/reader_output.go

@@ -0,0 +1,8 @@
+package main
+
+func (r Read2) Read(p []byte) (n int, err error) {
+	panic("not implemented") // TODO: Implement
+}
+
+
+type Read2 struct{}