unix: Add special function to improve coverage.

The function and corresponding command-line option are only enabled for
the coverage build.  They are used to exercise uPy features that can't
be properly tested by Python scripts.

unix/coverage.c

@@ -0,0 +1,26 @@
+#include <stdio.h>
+
+#include "py/obj.h"
+#include "py/runtime.h"
+#include "py/repl.h"
+
+#if defined(MICROPY_UNIX_COVERAGE)
+
+// function to run extra tests for things that can't be checked by scripts
+void run_extra_coverage_tests(void);
+
+void run_extra_coverage_tests(void) {
+    // repl autocomplete
+    {
+        const char *str;
+        mp_uint_t len = mp_repl_autocomplete("__", 2, &mp_plat_print, &str);
+        printf("%.*s\n", (int)len, str);
+
+        mp_store_global(MP_QSTR_sys, mp_import_name(MP_QSTR_sys, mp_const_none, MP_OBJ_NEW_SMALL_INT(0)));
+        mp_repl_autocomplete("sys.", 4, &mp_plat_print, &str);
+        len = mp_repl_autocomplete("sys.impl", 8, &mp_plat_print, &str);
+        printf("%.*s\n", (int)len, str);
+    }
+}
+
+#endif