feat: issue warning when gno test package can't be resolved (gnolang#…

…1845)

Pretty straightforward one line change. If `gno test` fails to find a
gno.mod field or package path in gno root, the result will be a panic
with the message `panic: unexpected unreal object`, which makes it
pretty difficult to figure out what must be done to correct the issue.
The warning gives a solution on how to fix this.

gnovm/cmd/gno/test.go

@@ -318,6 +318,7 @@ func gnoTestPkg(
 			gnoPkgPath = pkgPathFromRootDir(pkgPath, rootDir)
 			if gnoPkgPath == "" {
 				// unable to read pkgPath from gno.mod, generate a random realm path
+				io.ErrPrintfln("--- WARNING: unable to read package path from gno.mod or gno root directory; try creating a gno.mod file")
 				gnoPkgPath = transpiler.GnoRealmPkgsPrefixBefore + random.RandStr(8)
 			}
 		}