test: remove common.tmpDirName

`common.tmpDirName` is used in only one test and can be replaced with
`path.basename(common.tmpDir)`.

PR-URL: https://github.com/nodejs/node/pull/17266


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>