update assertions to use descriptive variable names

[sateeshperi]

@netlify /docs/contributing/nf-test/assertions

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[sateeshperi]

@SPPearce reminder abt this PR

[mashehu]

@SPPearce did you find the yaml code yet? 😉

[mashehu]

any updates on this one @SPPearce and @sateeshperi?

[sateeshperi]

lets bundle these changes into the nf-test guide