mtest: refactor common condition into a property of TestRun
mtest: refactor common condition into a property of TestRun

For a test to be displayed to stdout without buffering, it has to be
1) in verbose mode 2) not executed in parallel with others
3) not parsed, i.e. not TAP or Rust.  Include these three
conditions in a new property of stdout and use it in the
three places where it matters: when printing the initial
and final delimiters of the output, and when deciding the
console mode to use.

Suggested-by: Jussi Pakkanen <f02161d0fc5acca032e4116afd9b1eebea6af816@gmail.com>
Signed-off-by: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>

pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson