tests: use "returns_" rather than explicit comparison with "$?"

tests: use "returns_" rather than explicit comparison with "$?"

The previous "returns_"-using change failed to convert many
uses of "$?".  Convert all but two of the remaining ones.
* tests/ls/stat-vs-dirent.sh: Likewise.
* tests/misc/head-write-error.sh: Likewise.
* tests/misc/nice.sh: Likewise.
* tests/misc/nohup.sh: Likewise.
* tests/misc/stdbuf.sh: Likewise.
* tests/misc/sync.sh: Likewise.
* tests/tail-2/pid.sh: Likewise.
* tests/tail-2/wait.sh: Likewise.
Thanks to Bernhard Volker for spotting this.