devtools: reduce build test verbosity

devtools: reduce build test verbosity

The default verbosity of test-meson-builds.sh is to be quiet.
In order to better apply the verbosity policy, some file descriptors
are open to redirect to stdout or /dev/null accordingly.

The target variable and meson/ninja commands are printed in verbose modes.
The installation commands are printed only in very verbose mode.
The examples build commands are printed only in very verbose mode.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>