test(fixtures): add Arch package fixtures and prove Lua cross-validation probe

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

testdata/srcinfo/mytool/PKGBUILD

@@ -0,0 +1,20 @@
+pkgname=mytool
+pkgver=1.0
+pkgrel=1
+pkgdesc="Example custom-build package"
+arch=(x86_64)
+url="https://example.org"
+depends=('glibc')
+source=("mytool-1.0.tar.gz")
+sha256sums=('SKIP')
+
+build() {
+  cd "$srcdir"
+  ./configure --prefix=/usr
+  make
+}
+
+package() {
+  cd "$srcdir"
+  make DESTDIR="$pkgdir" install
+}