Reorganize wasi-misc-tests. (#575)

* Reorganize wasi-misc-tests.

Move wasi-misc-tests out of wasi-common, to break a dependency cycle;
previously, wasmtime-* depended on wasi-common, but wasi-common
dev-dependended on wasmtime-*.

Now, wasi-common no longer dev-depends on wasmtime-*; instead, the
tests are in their own crate which depends on wasi-common and on
wasmtime-*.

Also, rename wasi-misc-tests to wasi-tests for simplicity.

This also removes the "wasm_tests" feature; it's replaced by the
"test-programs" feature.

* Update the CI script to use the new feature name.

* Update the CI script to use the new feature name in one more place.

* Change a `write!` to a `writeln!`.

crates/test-programs/wasi-tests/src/bin/big_random_buf.rs

@@ -0,0 +1,18 @@
+use wasi::wasi_unstable;
+
+fn test_big_random_buf() {
+    let mut buf = Vec::new();
+    buf.resize(1024, 0);
+    assert!(
+        wasi_unstable::random_get(&mut buf).is_ok(),
+        "calling get_random on a large buffer"
+    );
+    // Chances are pretty good that at least *one* byte will be non-zero in
+    // any meaningful random function producing 1024 u8 values.
+    assert!(buf.iter().any(|x| *x != 0), "random_get returned all zeros");
+}
+
+fn main() {
+    // Run the tests.
+    test_big_random_buf()
+}