[NFC] wasmstdlib.py: fix Python typing error (#83125)
Fix type error in `should_test_executable`:
```
reportIncompatibleMethodOverride: Method "should_test_executable" overrides class "WasmStdlib" in an incompatible manner
Return type mismatch: base method returns type "Literal[True]", override returns type "Literal[False]"
"Literal[False]" is not assignable to type "Literal[True]"
```
