Fix panic with setInterval in k6/ws

[mstoykov]

The panic happens when interval between 0 and 1 is given.

The code that tested whether the interval was > 0 correctly converted to
floats first and then back to time. And then checked it in order to
prevent this very panic.

Unfortunately it then did the same calculation again but this time by
converting the interval directly to int before multiplying by
milliseconds. Which ends up rounding it to 0. Making the whole result
zero and leading to time.NewTicker to panic.

This was already done correctly for setTimeout when the same panic was
found there and the wrong conversation was added for setInterval.

Previous commit df3ad1b

[codecov-commenter]

Codecov Report

Merging #2896 (8484c98) into master (4e88cc1) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 8484c98 differs from pull request most recent head a444065. Consider uploading reports for the commit a444065 to get more accurate results

@@            Coverage Diff             @@
##           master    #2896      +/-   ##
==========================================
- Coverage   76.93%   76.87%   -0.06%     
==========================================
  Files         225      223       -2     
  Lines       16867    16862       -5     
==========================================
- Hits        12976    12963      -13     
- Misses       3060     3065       +5     
- Partials      831      834       +3     

Flag	Coverage Δ
ubuntu	76.87% <100.00%> (+0.02%)	⬆️
windows	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
js/modules/k6/ws/ws.go	86.08% <100.00%> (ø)
loader/filesystems.go	60.00% <0.00%> (-40.00%)	⬇️
js/common/initenv.go	66.66% <0.00%> (-33.34%)	⬇️
api/v1/metric.go	77.27% <0.00%> (-4.55%)	⬇️
cmd/tests/test_state.go	87.75% <0.00%> (-4.09%)	⬇️
metrics/metric_type.go	78.94% <0.00%> (-3.51%)	⬇️
lib/netext/httpext/error_codes.go	94.52% <0.00%> (-2.74%)	⬇️
lib/netext/httpext/error_codes_syscall_windows.go
cmd/ui_windows.go
lib/executor/vu_handle.go	95.32% <0.00%> (+0.93%)	⬆️
... and 1 more

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