curl: rewrite ptest installation

The latest libtool upgrade appears to have resulted in intermediate files
containing build paths. This wouldn't normally be a problem but the
curl-ptest package is populated by copying ${B}/tests/ which includes all
of the intermediate objects, so this causes buildpath warnings.

Rewrite the ptest installation to install just the pieces we need: the
test harness, utility scripts, and the test data.  We do not need the
libcurl unit tests nor the HTTP server tests, as we don't run those.

Remove all of the explicitly disabled tests, as many of these were for
tests that run curl-config or scan the source code, neither of which are
available at ptest time.  Instead use keyword exclusions to skip them.

Tell the test runner to use the system curl instead of symlinking in the
binary.

Don't copy curl-config, skipping these minor tests is acceptable.

Remove the RDEPENDS on bash, nothing needs this now we're not shipping a
build tree.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>