tests: use more POSIX shell features our test scripts

tests: use more POSIX shell features our test scripts

Since commit 'v1.12-36-g2d68fd9' of 2012-05-07, "configure: search a
sturdy POSIX shell to be used in the testsuite", the shell running
our test script is assured to be a POSIX-conforming shell, so we can
use the more modern and flexible idioms and features that we couldn't
use when we also aimed at compatibility with non-POSIX Bourne shells,
like Solaris /bin/sh.

* t/README: Suggest to use POSIX shell features liberally in test cases,
with possible exception of Makefile recipes and configure shell code.
* Several tests: Adjust to use more POSIX shell features; e.g., $(...)
rather than `...`, $((...)) rather than `expr ...`, "if ! CMD; then ..."
instead of "if CMD; then :; else ...", and so on.
In several places, when using the 'test' built-in, prefer '-eq' over
'=' for numeric comparisons, and prefer "grep -c PATTERN FILE" over
"grep PATTERN FILE | wc -l".
Throw in other low-hanging easy improvements and fixlets while we are
at it.
* t/ax/depcomp.sh, t/ax/tap-summary-aux.sh, t/ax/tap-functions.sh,
defs, defs-static.in: Likewise.

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