Fix new shellcheck warnings with shellcheck v0.7.2
The prefixed "x" construct within tests was used due to ancient bogus shell implementations, but is no longer relevant. Fixes SC2268 warnings with shellcheck v0.7.2.
Disable SC2030 + SC2031 issues in relevant lines in unmkinitramfs. Those are false positives which are fixed with an upcoming version of shellcheck, see https://github.com/koalaman/shellcheck/issues/2217
Closes: #992798