tests: move TESTCASES to Makefile.inc, add show for cmake

This change allows runtests.pl to be run from the CMake builddir:

    export srcdir=/tmp/curl/tests;
    perl -I$srcdir $srcdir/runtests.pl -l

In order to make this possible, all test cases have been moved from
Makefile.am to Makefile.inc.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>