storage test: use new file for testTouchCreateFile()

this test would never succeed, because the previous test -
testFOpen() - creates the file 'foo', but testTouchCreateFile()
starts out by asserting it doesn't exist. Change the test to
use a file called 'touch' instead (which does not previously
exist).
Merge pull request #12093 from AdamWill/storage-test-touch

storage test: use new file for testTouchCreateFile()