testsuite: fix spurious failures with Solaris /bin/sh

testsuite: fix spurious failures with Solaris /bin/sh

* tests/tap-more.test: Use `echo > file', not `: > file', to
create empty files in the "for" loops; this is required since,
as documented in the autoconf manual, Solaris 10 /bin/sh
"optimizes" away the `:' command after the first iteration,
even if it is redirected.
* tests/test-driver-custom-multitest-recheck2.test: Likewise.
* tests/tap-numeric-description.test: Partial rewrite to avoid
using positional parameters from the 10th onward, which are
unportable to Solaris /bin/sh (using `${10}' causes the shell
to die with "bad substitution").