examples: Add missing Close calls

[alexandear]

The PR fixes sqltest.SQLite usages and implementation by adding the missing sdb.Close() and source.Close().

sqltest.SQLite returns *sql.DB, which we need to close as shown here:

sqlc/internal/endtoend/vet_test.go

Lines 57 to 59 in 9cf5dbc

	db, cleanup := sqltest.CreateSQLiteDatabase(t, dsn, []string{s})
	defer db.Close()
	defer cleanup()

os.CreateTemp creates a new temporary file in the directory dir, opens the file for reading and writing, and returns the resulting file. We also need to close it.

[kyleconroy]

Thanks!