Add tests using testdata input files

- add collection of testdata files
  - `input`
    - `encoded-all`
    - `encoded-mixed`
    - `invalid-all`
    - `invalid-mixed`
  - `output`
    - `embedded`
    - `standalone`
- add "exported" tests which focus strictly on exported functionality
  of the `internal/safelinks` package
- add minor dependency on the `google/go-cmp` package for some tests
- add brief README for testdata content to explain purpose, important
  of preserving the exact formatting (mostly specific EOL) within the
  input and output testdata files

refs GH-251

go.mod

@@ -2,7 +2,10 @@ module github.com/atc0005/safelinks
 
 go 1.20
 
-require fyne.io/fyne/v2 v2.4.4
+require (
+	fyne.io/fyne/v2 v2.4.4
+	github.com/google/go-cmp v0.6.0
+)
 
 require (
 	fyne.io/systray v1.10.1-0.20231115130155-104f5ef7839e // indirect

go.sum

@@ -144,6 +144,8 @@ github.com/google/go-cmp v0.5.3/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/
 github.com/google/go-cmp v0.5.4/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-cmp v0.5.5/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-cmp v0.5.6/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
+github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/google/gofuzz v1.0.0/go.mod h1:dBl0BpW6vV/+mYPU4Po3pmUjxk6FQPldtuIdl/M65Eg=
 github.com/google/martian v2.1.0+incompatible/go.mod h1:9I4somxYTbIHy5NJKHRl3wXiIaQGbYVAs8BPL6v8lEs=
 github.com/google/martian/v3 v3.0.0/go.mod h1:y5Zk1BBys9G+gd6Jrk0W3cC1+ELVxBWuIGO+w/tUAp0=