frontend: fix testsuite stderr warnings
helpers.py:14: SyntaxWarning: "is" with a literal. Did you mean "=="?
if type is "10min":
helpers.py:18: SyntaxWarning: "is" with a literal. Did you mean "=="?
elif type is "30min":
helpers.py:22: SyntaxWarning: "is" with a literal. Did you mean "=="?
elif type is "24h":