devtools: control location of test builds

devtools: control location of test builds

By default, both test-build.sh and test-meson-builds.sh scripts create the
builds they generate in the current working directory, leading to a large
number of build directories being present when testing patches. This
patchset modifies both scripts to use a DPDK_BUILD_TEST_DIR environment
variable to control where the build outputs are put.

For example, doing:
    export DPDK_BUILD_TEST_DIR=__builds
    ./devtools/test-meson-builds.sh && ./devtools/test-build.sh \
            x86_64-native-linux-clang+shared i686-native-linux-gcc

gives a "__builds" directory with 14 meson and 2 make builds (with the
meson build count depending on compiler availability)

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