test: apply no-useless-concat ESLint rule
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
test
It seems to me that no-useless-concat is a simple reasonable rule, so I've tried:
eslint --rulesdir tools/eslint-rules/ --rule "no-useless-concat: error" benchmark lib test tools
which have found only some fragments in tests. Occasionally, I've added some template literals for simplicity, but that was not the main aim. Please, take a look, if this is a reasonable change.