tst_QWebEngineProfile::changePersistentPath is flaky

agent:2020/04/21 01:43:41 build.go:253: FAIL! : tst_QWebEngineProfile::changePersistentPath() 'server.stop()' returned FALSE. ()
agent:2020/04/21 01:43:41 build.go:253: Loc: [tst_qwebengineprofile.cpp(879)]

Since the build is still ongoing and I don't have a link to testresults, I'll provide the commit that will have the link to full logs: https://codereview.qt-project.org/c/qt/qt5/+/297681

QtMetrics also confirms that this autotest has failed once in macOS 10.13 and twice in Ubuntu 18.04 during the last 6 months: https://testresults.qt.io/grafana/d/000000009/coin-single-test-details?orgId=1&var-project=qt%2Fqtwebengine&var-testcase=tests%2Fauto%2Fwidgets%2Fqwebengineprofile&var-testfunction=All&var-branch=dev&var-branch=5.15&var-branch=5.14&var-branch=5.13&var-branch=5.12&var-result=Failed&var-inter=24h&from=1571605200000&to=1587416400000