When a `distcheck` fails, the $(distdir) is not removed and this is how
we can collect test results. This directory is also left untouched by
the `distclean` target run from `autogen.des`.

So if the next commit checked by vtest fails before running inside the
$(distdir), it will report failures from the previous run. To avoid that
we can use $(am__remove_distdir) that deals with permissions but may not
be stable API.

Instead of randomly looking for the log files, look for them where we
expect them to land. For instance, after a git pull, a distclean
operation may not remove the dist directory if Varnish's version
changed because it would reconfigure itself on the fly.

Instead of reading the output of `make distcheck`, look for failing
tests in the dist directory.

to avoid any memory leaks.

Closes	#2177

Unlike other sets of commands, function names don't start with
`cmd_stream_` but I didn't bulk-rename them. It can be done later
if it's really that important.

With this we can move body checks closer to HTTP checks.

Commands are sorted alphabetically inside categories. A bunch of things
had to be renamed to fit into a pattern.

In OSX with the Xcode toolchain DWARF debug symbol files (.dSYM) will be
created when cc_command is executed (when -g is present) and the directory
will not be empty.

Prompted after b2bee0e3.

Move its remainings to libvarnishapi and include them conditionally.

VMODs.  Otherwise jail_unix doesn't allow you to use VMODs.

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.