Fix testProjectDetailsLastAccess

this test failed most of the time since gsoc_backpack merge - https://jenkins.catrob.at/job/DeviceTest/597/testReport/org.catrobat.catroid.uitest.ui.activity/MyProjectsActivityTest/testProjectDetailsLastAccess/history/?

two problems caused this failure: sometimes there were only 2 instead of 3 projects visible, but the test checks for today, yesterday and 1st january as last modification time.
and sometimes, since programs are saved within asynctask when switching from main to programlist, modificationtime was overwritten if the asynctask took longer than expected!
removing UiTestUtils.goBackToHome solved a possible problem with IOException which occurred in MyProjectsActivityTest cases - https://jenkins.catrob.at/view/Catroid/job/Catroid-single-UI-device/111/artifact/logcat.txt

multirun: https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/506/ (seems that webserver was down)
single run of failing testcase (testEqualLookNames): https://jenkins.catrob.at/view/Catroid/job/Catroid-single-UI-emulator/283/
single run with 20 iterations (testProjectDetails + testProjectDetailsLastAccess): https://jenkins.catrob.at/view/Catroid/job/Catroid-single-UI-device/115/console