tester: improve API surface and implement one more test function in lis2mdl driver

Signed-off-by: deadprogram <ron@hybridgroup.com>

tester/tester.go

@@ -3,3 +3,11 @@
 // TODO: info on how to use this.
 //
 package tester // import "tinygo.org/x/drivers/tester"
+
+// Failer is used by the I2CDevice type to abort when it's used in
+// unexpected ways, such as reading an out-of-range register.
+type Failer interface {
+	// Fatalf prints the Printf-formatted message and exits the current
+	// goroutine.
+	Fatalf(f string, a ...interface{})
+}