tests/ports/esp32/check_err_str.py: Preallocate global variable.

This test fails on all esp32 boards without this fix, because the
try/except that runs with the heap locked attempts to increase the size of
the globals dict when assigning to the exception variable `e`.

Fix that by preallocating the global variable `e`.

Signed-off-by: Damien George <damien@micropython.org>

tests/ports/esp32/check_err_str.py

@@ -1,3 +1,5 @@
+# This tests checks the behaviour of the `check_esp_err`/`check_esp_err_` C function.
+
 try:
     from esp32 import Partition as p
     import micropython
@@ -23,6 +25,7 @@ except OSError as e:
 
 # same but with out of memory condition by locking the heap
 exc = "FAILED TO RAISE"
+e = None  # preallocate entry in globals dict
 micropython.heap_lock()
 try:
     fun(part)
@@ -34,6 +37,7 @@ print("exc:", exc)  # exc empty due to no memory
 # same again but having an emergency buffer
 micropython.alloc_emergency_exception_buf(256)
 exc = "FAILED TO RAISE"
+e = None  # preallocate entry in globals dict
 micropython.heap_lock()
 try:
     fun(part)