test: improve the code in test-fs-read-stream

[edsadr]

• use const and let instead of var
• use common.mustCall to control function execution
• use assert.strictEqual instead of assert.equal
• use arrow functions

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test