test: correct TEST-41 StartLimitBurst test

test: correct TEST-41 StartLimitBurst test

TEST-41 verifies that the StartLimitBurst property will correctly
limit the number of unit restarts, but the test currently doesn't
adjust the StartLimitIntervalSec which defaults to 10 seconds.

On Ubuntu CI, running under un-accelerated qemu, it can take more than
10 seconds to perform all 3 restarts, which avoids the burst limit,
and fails the test.

Instead, specify a long StartLimitIntervalSec in the test, so we can
be sure to correctly test StartLimitBurst even on slow testbeds.

Fixes #13794.