testsuite: Fix Executable-Relocatable test package database location

We were attempting to register a package into the global package
database, which then triggered some other check to do with relocatable
packages to fail.

The fix is to initialise and use a local package database, so the
package would be built and installed into the local package database.

cabal-testsuite/PackageTests/PathsModule/Executable-Relocatable/setup.test.hs

@@ -4,4 +4,4 @@ import Test.Cabal.Prelude
 main = setupAndCabalTest $ do
   skipIfWindows
   skipUnlessGhcVersion ">= 8.0"
-  setup_build ["--enable-relocatable"]
+  withPackageDb $ setup_build ["--enable-relocatable"]