[INTERNAL] sap.m.ComboBox: add more QUnit-tests for onsapup/onsapdown

- the typed value should not gets selected when when arrow DOWN and UP
keys are pressed

Change-Id: I551de1a01b60ccff3b161dd15b7766b0cf5dc8ce
Merge "[INTERNAL] sap.m.ComboBox: add more QUnit-tests for onsapup/onsapdown"