Convert test_zzz_zzz_emmalloc_4gb to btest_exit (#20656)

This simplifies the test so it can also be run under node. e.g.

```
$ ./test/runner browser.test_zzz_zzz_emmalloc_4gb --browser=node
```

Author: sbc100

test/mem_growth.cpp

@@ -5438,10 +5438,10 @@ def test_wasm_worker_proxied_function(self):
   @requires_v8
   def test_zzz_zzz_4gb(self):
     # TODO Convert to an actual browser test when it reaches stable.
-    #      For now, keep this in browser as this suite runs serially, which
-    #      means we don't compete for memory with anything else (and run it
-    #      at the very very end, to reduce the risk of it OOM-killing the
-    #      browser).
+    # For now, keep this in browser as this suite runs serially, which
+    # means we don't compete for memory with anything else (and run it
+    # at the very very end, to reduce the risk of it OOM-killing the
+    # browser).
 
     # test that we can allocate in the 2-4GB range, if we enable growth and
     # set the max appropriately
@@ -5451,9 +5451,11 @@ def test_zzz_zzz_4gb(self):
   # Tests that emmalloc supports up to 4GB Wasm heaps.
   @no_firefox('no 4GB support yet')
   def test_zzz_zzz_emmalloc_4gb(self):
-    self.btest('mem_growth.cpp',
-               expected='-65536', # == 4*1024*1024*1024 - 65536 casted to signed
-               args=['-sMALLOC=emmalloc', '-sABORTING_MALLOC=0', '-sALLOW_MEMORY_GROWTH=1', '-sMAXIMUM_MEMORY=4GB'])
+    # For now, keep this in browser as this suite runs serially, which
+    # means we don't compete for memory with anything else (and run it
+    # at the very very end, to reduce the risk of it OOM-killing the
+    # browser).
+    self.btest_exit('test_mem_growth.c', args=['-sMALLOC=emmalloc', '-sABORTING_MALLOC=0', '-sALLOW_MEMORY_GROWTH=1', '-sMAXIMUM_MEMORY=4GB'])
 
   # Test that it is possible to malloc() a huge 3GB memory block in 4GB mode using emmalloc.
   # Also test emmalloc-memvalidate and emmalloc-memvalidate-verbose build configurations.

test/test_browser.py

@@ -0,0 +1,28 @@
+#include <assert.h>
+#include <stdio.h>
+
+#include <emscripten/emmalloc.h>
+#include <emscripten/html5.h>
+#include <emscripten/heap.h>
+
+#define EXPECTED_FINAL_HEAP_SIZE (4*1024*1024*1024ll - 65536)
+
+int main() {
+  size_t prevheapsize = 0;
+  int count = 0;
+  while (1) {
+    size_t heap_size = emscripten_get_heap_size();
+    if (prevheapsize != heap_size) {
+      printf("Heap size: %zu inc: %zu count: %d\n", heap_size, heap_size - prevheapsize, count);
+    }
+
+    prevheapsize = heap_size;
+    void *ptr = malloc(16*1024*1024);
+    if (!ptr) {
+      printf("Cannot malloc anymore. Final heap size: %zu\n", emscripten_get_heap_size());
+      assert(emscripten_get_heap_size() == EXPECTED_FINAL_HEAP_SIZE);
+      return 0;
+    }
+    count++;
+  }
+}