test: refactor test-http-client-readable

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test http

Description of change

• var -> const
• remove function names where V8 inference is as good or better
• add function names where there is no V8 inference
• assert.equal -> strictEqual
• move assertion from exit handler to response end handler