qemu-arm: Reinitialise uPy heap and runtime at start of each test.

Previous to this patch, all qemu-arm tests were running in the same
session, and global variables could be left over from the previous test.
This patch makes it so that the heap and runtime are reinitialised at
the start of each test.

qemu-arm/test_main.c

@@ -15,8 +15,14 @@
 #include "tinytest.h"
 #include "tinytest_macros.h"
 
+#define HEAP_SIZE (128 * 1024)
+STATIC void *heap;
+
 void do_str(const char *src);
 inline void do_str(const char *src) {
+    gc_init(heap, (char*)heap + HEAP_SIZE);
+    mp_init();
+
     mp_lexer_t *lex = mp_lexer_new_from_str_len(MP_QSTR__lt_stdin_gt_, src, strlen(src), 0);
     if (lex == NULL) {
         tt_abort_msg("Lexer initialization error");
@@ -36,13 +42,13 @@ inline void do_str(const char *src) {
             // TODO: That can be always true, we should set up convention to
             // use specific exit code as skip indicator.
             tinytest_set_test_skipped_();
-            return;
+            goto end;
         }
         mp_obj_print_exception(&mp_plat_print, exc);
         tt_abort_msg("Uncaught exception");
     }
 end:
-    ;
+    mp_deinit();
 }
 
 #include "genhdr/tests.h"
@@ -51,11 +57,8 @@ int main() {
     const char a[] = {"sim"};
     mp_stack_ctrl_init();
     mp_stack_set_limit(10240);
-    void *heap = malloc(256 * 1024);
-    gc_init(heap, (char*)heap + 256 * 1024);
-    mp_init();
+    heap = malloc(HEAP_SIZE);
     int r = tinytest_main(1, (const char **) a, groups);
-    mp_deinit();
     printf( "status: %i\n", r);
     return r;
 }