Decrease default wasm stack to 512k from 1M (#3861)

This commit aims to achieve the goal of being able to run the test suite
on Windows with `--test-threads 1`, or more notably allowing Wasmtime's
defaults to work better with the main thread on Windows which appears to
have a smaller stack by default than Linux by comparison. In decreasing
the default wasm stack size a test is also update to probe for less
stack to work on Windows' main thread by default, ideally allowing the
full test suite to work with `--test-threads 1` (although this isn't
added to CI as it's not really critical).

Closes #3857

tests/all/stack_overflow.rs

@@ -4,8 +4,8 @@ use wasmtime::*;
 #[test]
 fn host_always_has_some_stack() -> anyhow::Result<()> {
     static HITS: AtomicUsize = AtomicUsize::new(0);
-    // assume hosts always have at least 512k of stack
-    const HOST_STACK: usize = 512 * 1024;
+    // assume hosts always have at least 128k of stack
+    const HOST_STACK: usize = 128 * 1024;
 
     let mut store = Store::<()>::default();