devtools: rename build test verbosity variables

devtools: rename build test verbosity variables

The verbosity was meant to be set with options -v and -vv,
or possibly with the environment variables TEST_MESON_BUILD_VERBOSE
and TEST_MESON_BUILD_VERY_VERBOSE.

It is decided to keep only the options -v and -vv,
so the variables are renamed with lower case, marking them as privates.

The handling of the verbosity level is also moved upper in the script,
closer to other initializations.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: David Marchand <david.marchand@redhat.com>