Lexicographic order is fine, but can lead to interesting things like:

    ================================================================
    Testsuite summary for Varnish trunk
    ================================================================
    # TOTAL: 571
    # PASS:  565
    # SKIP:  6
    # XFAIL: 0
    # FAIL:  0
    # XPASS: 0
    # ERROR: 0
    ================================================================
    Making check in varnishtop
      CC       varnishtop-varnishtop.o
      CC       varnishtop-varnishtop_options.o
      CCLD     varnishtop
    Making check in etc
    Making check in doc
    Making check in graphviz
    Making check in sphinx
    Making check in man

In other words, picking `varnishtop` from somewhere else in the PATH.
Since SUBDIRS are processed sequentially, this is change is enough to
cover parallel builds too.

While at it, polish u4.vtc a bit: get rid of the shell process.