detection of missing packages in test-all

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Dear developers,

Following works of Loïc Grenié at the Ateliers, I have changed dotest
to skip tests that require packages that are not available.
This report the list of skipped tests at the end.

For example, if galdata is not available:
make test-galois
Making test-galois in Olinux-x86_64
make[1]: entrant dans le répertoire « /home/bill/pari/Olinux-x86_64 »
Skipping galois because optional package galdata has not been installed.
+++ Total bench for gp-sta is 0
+++ Total bench for gp-dyn is 0
Note: the following tests have been skipped: galois
make[1]: quittant le répertoire « /home/bill/pari/Olinux-x86_64 »

The syntax is as follow: at the start of the test file, a comment should be added
like
\\package:galdata
that lists required packages (omit it if none).

Cheers,
Bill.