tests: remove stale workarounds for Solaris /bin/sh

tests: remove stale workarounds for Solaris /bin/sh

Our testsuite cannot be run with that shell anymore (as it is
not POSIX-compliant).

* t/tap-more.sh: Remove outdated workarounds for Solaris /bin/sh.
* t/self-check-exit.tap: Likewise.
* t/ansi2knr-no-more.sh: Likewise.
* t/add-missing.tap: Likewise.
* t/dist-auxfile.sh: Likewise.
* t/test-driver-custom-multitest-recheck2.sh:  Likewise.
* t/ax/test-init.sh: Remove obsolete references to Solaris
/bin/sh.
* t/confh5.sh: Likewise.
* t/uninstall-fail.sh: Likewise.  And update comments about
quirks of Solaris /bin/ksh and /usr/xpg4/bin/sh.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>