fix(gnovm): correct filetest directive behavior (#3697)

The test result is misleading. Without an output directive set, the
current logic does not provide a prompt even when actual output occurs.

```go
package main

func main() {
	println("ok")
}

// Error:
// panic xxx
```

## before fix:

=== RUN   TestFiles
=== PAUSE TestFiles
=== CONT  TestFiles
=== RUN   TestFiles/a111.gno
--- PASS: TestFiles (0.14s)
    --- PASS: TestFiles/a111.gno (0.01s)
PASS
ok  	command-line-arguments	1.619s


## after fix:

=== RUN   TestFiles
=== PAUSE TestFiles
=== CONT  TestFiles
=== RUN   TestFiles/a111.gno
    files_test.go:92: unexpected output:
        ok

--- FAIL: TestFiles (0.13s)
    --- FAIL: TestFiles/a111.gno (0.01s)
FAIL
FAIL	command-line-arguments	1.479s
FAIL

---------

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

gnovm/pkg/test/filetest.go

@@ -103,6 +103,11 @@ func (opts *TestOptions) runFiletest(filename string, source []byte) (string, er
 		// The Error directive (and many others) will have one trailing newline,
 		// which is not in the output - so add it there.
 		match(errDirective, result.Error+"\n")
+	} else if result.Output != "" {
+		outputDirective := dirs.First(DirectiveOutput)
+		if outputDirective == nil {
+			return "", fmt.Errorf("unexpected output:\n%s", result.Output)
+		}
 	} else {
 		err = m.CheckEmpty()
 		if err != nil {

gnovm/tests/files/addressable_5.gno

@@ -9,3 +9,6 @@ func main() {
 	le := &binary.LittleEndian
 	println(&le.AppendUint16(b, 0)[0])
 }
+
+// Output:
+// &(0 <databyte> uint8)

gnovm/tests/files/type_alias.gno

@@ -6,7 +6,8 @@ import "gno.land/p/demo/uassert"
 type TestingT = uassert.TestingT
 
 func main() {
-	println(TestingT)
+	println("ok")
 }
 
-// No need for output; not panicking is passing.
+// Output:
+// ok

gnovm/tests/files/types/eql_0f49.gno

@@ -14,6 +14,8 @@ func main() {
 
 }
 
+// Output:
+// true
 // true
 // true
 // true