Embed testdata/flatgeobuf/*.fgb in example tests

The purpose of this change is to see if it will enable the example
tests to run on https://pkg.go.dev/github.com/gogama/flatgeobuf.

Before this change, at least, they are failing with errors like the one
shown below. If this change isn't successful, I will most likely revert
it and perhaps resort to some other form of embedding, like directly
putting the FGB bytes in a []byte within `example_test.go`.

```
panic: open testdata/flatgeobuf/empty.fgb: no such file or directory

goroutine 1 [running]:
main.openFile({0x4a7312?, 0x40b33e?})
	/tmp/sandbox2858312298/prog.go:13 +0x3c
main.main()
	/tmp/sandbox2858312298/prog.go:24 +0x3d
```

Author: vcschapp

TODO.md

@@ -9,7 +9,8 @@ NEXT STEPS:
     4. Cut v0.9.5-beta.
     5. Add GitHub action to build/test on commit? (If free.)
     6. Update copyrights to 2024. 
-    7. Cut v1.0.0.
+    7. Clear out any remaining TODO, for e.g. example_test.go. 
+    8. Cut v1.0.0.
 
 Future directions:
 1. Another interesting interaction system would be an Appender which

flatgeobuf/example_test.go

@@ -6,9 +6,10 @@ package flatgeobuf_test
 
 import (
 	"bytes"
+	"embed"
 	"encoding/hex"
 	"fmt"
-	"os"
+	"io/fs"
 	"sort"
 
 	"github.com/gogama/flatgeobuf/flatgeobuf"
@@ -17,8 +18,11 @@ import (
 	flatbuffers "github.com/google/flatbuffers/go"
 )
 
-func openFile(name string) *os.File {
-	f, err := os.Open(name)
+//go:embed testdata/flatgeobuf/*.fgb
+var testFiles embed.FS
+
+func openFile(name string) fs.File {
+	f, err := testFiles.Open(name)
 	if err != nil {
 		panic(err)
 	}