python3: parallelize ptests, add test_cppext dependencies

python3: parallelize ptests, add test_cppext dependencies

This is based on a previous patch from Alex Kanavin (thanks Alex),
but it retains our use of sed to parse the test output into a usable
form. The intent was to use the test module's --junit-xml feature and
parse the resulting log instead of using sed, but various errors were
encountered when testing this method, so that will need further
investigation before we can consider adopting it. Two missing ptest
dependencies on python3-cgitb and python3-zipapp were already merged
in a previous patch, so only gcc, g++, and binutils were left to add
as ptest RDEPENDS.

ptest output and runtime with this change:

== Tests result: SUCCESS ==

405 tests OK.

29 tests skipped:
    test_asdl_parser test_check_c_globals test_clinic test_curses
    test_devpoll test_gdb test_idle test_ioctl test_kqueue
    test_launcher test_msilib test_ossaudiodev test_readline
    test_smtpnet test_socketserver test_startfile test_tcl test_tix
    test_tk test_ttk_guionly test_ttk_textonly test_turtle
    test_urllib2net test_urllibnet test_winconsoleio test_winreg
    test_winsound test_xmlrpc_net test_zipfile64

Total duration: 5 min 3 sec

[YOCTO #13298]

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>