tests: don't hardcode sudo implementation

Remove the SUDO environment variable, as it depends on what's installed
and configured on the host what implementation should be used.

It would not suffice to add sudo or doas as a makedepend, as it would
also need to be configured to allow abuild-keygen -ain to be executed
without password.

tests/abuild-sign.bats

@@ -10,7 +10,6 @@ setup() {
 	export WORKDIR="$tmpdir"/work
 	export APORTSDIR="$PWD"/testrepo
 	export PATH="$PWD/../:$PATH"
-	export SUDO=doas
 	export ARCH=$(apk --print-arch)
 
 	abuild-keygen -ain >/dev/null 2>&1

tests/abuild.bats

@@ -11,7 +11,6 @@ setup() {
 	export GIT_CONFIG_GLOBAL="$tmpdir"/gitconfig
 	export APORTSDIR="$PWD"/testrepo
 	export PATH="$PWD/../:$PATH"
-	export SUDO=doas
 
 	abuild-keygen -ain >/dev/null 2>&1