tests: only generate key when not available yet

Installing a build key requires root permissions, something that is not
possible in all cases, most notably on the builders.

Instead of generating a key each time, which results in many keys being
generated each test suite run, check whether a key is already present by
using `abuild-sign --installed`, and only generate a key when that
returns false.

tests/abuild-sign.bats

@@ -12,7 +12,9 @@ setup() {
 	export PATH="$PWD/../:$PATH"
 	export ARCH=$(apk --print-arch)
 
-	abuild-keygen -ain >/dev/null 2>&1
+	if ! abuild-sign --installed 2>/dev/null; then
+		abuild-keygen -ain >/dev/null 2>&1
+	fi
 
 	mkdir -p "$tmpdir" "$WORKDIR"
 }

tests/abuild.bats

@@ -12,7 +12,9 @@ setup() {
 	export APORTSDIR="$PWD"/testrepo
 	export PATH="$PWD/../:$PATH"
 
-	abuild-keygen -ain >/dev/null 2>&1
+	if ! abuild-sign --installed 2>/dev/null; then
+		abuild-keygen -ain >/dev/null 2>&1
+	fi
 
 	mkdir -p "$tmpdir" "$WORKDIR"
 	printf "[color]\n\tui = always\n" > "$GIT_CONFIG_GLOBAL"