test: update coverage for dgram.js
Create test-dgram-multicast-loopback.js for dgram.setMulticastLoopback. Add a test which has an argument of an invalid number to test-dgram-setTTL.js. https://github.com/nodejs/node/blob/master/lib/dgram.js#L464 Add a test which has an argument of a string to test-dgram-multicast-setTTL.js. https://github.com/nodejs/node/blob/master/lib/dgram.js#L473
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines
Affected core subsystem(s)
test