support/testing: check if the defconfig provided for testing is valid

support/testing: check if the defconfig provided for testing is valid

Currently, the build continue even if some symbols disapear from
the generated dot config file (.config).

This patch add a new check in order to stop the test if one
of the provided symbol is missing. This must be treated as error.

For example, if a symbol disapear due to new dependency constraints.

Inspired by is_toolchain_usable() function from genrandconfig:
https://git.busybox.net/buildroot/tree/utils/genrandconfig?h=2020.02#n164

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>