test: add assertions for TextEncoder/Decoder
Added some assertions for TextEncoder/Decoder to expand branch coverage(https://coverage.nodejs.org/coverage-1385e1bc63665b8c/root/internal/encoding.js.html).
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