tests: Fix flaky test_that_press_reads_default_delay_from_stbt_conf
tests: Fix flaky test_that_press_reads_default_delay_from_stbt_conf

It was failing intermittently with:

    Expected: >= 0:00:00.5,
            got: 0:00:00.495523 between presses

It's because we can't control how much time elapses between these two
lines in the test:

    stbt.press('OK')
    time1 = datetime.datetime.now()

A tolerance of 0.05s will hopefully be enough.

stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester