tests: add missing test for invalid filename

fixes commit f35f2aa859 (abuild: add test for invalid filenames)

tests/testrepo/invalid-filename/APKBUILD

@@ -0,0 +1,28 @@
+# Maintainer: Natanael Copa <ncopa@alpinelinux.org>
+
+# test package
+pkgname="invalid-filename"
+pkgver="1.0"
+pkgrel=0
+pkgdesc="Dummy test package that fails to build"
+url="https://gitlab.alpinelinux.org/alpine/aports"
+arch="noarch"
+license="MIT"
+
+prepare() {
+	mkdir -p "$builddir"
+}
+
+build() {
+	touch $'bad\nfile'
+}
+
+check() {
+	true
+}
+
+package() {
+	mkdir -p "$pkgdir"
+	cp -r * "$pkgdir"/
+}
+