test: add else and error case for TextDecoder

[lauripiisang]

• add test for https://coverage.nodejs.org/coverage-17e4213987c6ad8d/root/internal/encoding.js.html#L365 missing else path
• add test for https://coverage.nodejs.org/coverage-17e4213987c6ad8d/root/internal/encoding.js.html#L385 missing error case

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[lauripiisang]

@jasnell : LGTM

[addaleax]

CI: https://ci.nodejs.org/job/node-test-pull-request/18372/

[jasnell]

Confirming my LGTM

[gireeshpunathil]

landed as 1d9ba7b

thank you @lauripiisang for the contribution! Wish you great success with continued contribution to this project, if you are further interested please have a look at https://www.nodetodo.org/next-steps