mtd: tests: check erase block count in page test

When there is only a single erase block, the cross erase test
does not report sensible errors. Warn in case there is only
a single erase block instead of executing the test.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>