Use strftime for formatting datetimes in test form submissions
Use strftime for formatting datetimes in test form submissions

Datetime-related tests are intermittently failing on Appveyor; the error at https://ci.appveyor.com/project/wagtail/wagtail/build/1.0.141/job/30t7t1ydh78gupie implies that this is due to invalid datetime formats being returned from submittable_timestamp. Using strftime means that we're not reliant on the return value of str(timestamp) being formatted in a specific way.

mixxorz/wagtail,mikedingjan/wagtail,rsalmaso/wagtail,iansprice/wagtail,mixxorz/wagtail,FlipperPA/wagtail,timorieber/wagtail,nimasmi/wagtail,torchbox/wagtail,FlipperPA/wagtail,zerolab/wagtail,torchbox/wagtail,rsalmaso/wagtail,gasman/wagtail,FlipperPA/wagtail,iansprice/wagtail,thenewguy/wagtail,gasman/wagtail,wagtail/wagtail,kaedroho/wagtail,wagtail/wagtail,kaedroho/wagtail,timorieber/wagtail,gasman/wagtail,zerolab/wagtail,nimasmi/wagtail,mixxorz/wagtail,jnns/wagtail,zerolab/wagtail,rsalmaso/wagtail,rsalmaso/wagtail,kaedroho/wagtail,thenewguy/wagtail,iansprice/wagtail,FlipperPA/wagtail,nealtodd/wagtail,jnns/wagtail,jnns/wagtail,mikedingjan/wagtail,thenewguy/wagtail,wagtail/wagtail,wagtail/wagtail,zerolab/wagtail,torchbox/wagtail,timorieber/wagtail,iansprice/wagtail,takeflight/wagtail,gasman/wagtail,wagtail/wagtail,torchbox/wagtail,nealtodd/wagtail,mikedingjan/wagtail,mikedingjan/wagtail,mixxorz/wagtail,nealtodd/wagtail,timorieber/wagtail,kaedroho/wagtail,nimasmi/wagtail,mixxorz/wagtail,thenewguy/wagtail,jnns/wagtail,nealtodd/wagtail,takeflight/wagtail,thenewguy/wagtail,zerolab/wagtail,rsalmaso/wagtail,takeflight/wagtail,gasman/wagtail,takeflight/wagtail,nimasmi/wagtail,kaedroho/wagtail