tests/run-natmodtests.py: Allow injected code customisation.

This commit introduces a mechanism to customise the code that is
injected to the board when performing a native module import.

A new argument, "-b"/"--begin", is added so regular Python code can be
inserted in the injected fragment between the module file creation and
the effective module import.  This is needed for running natmod tests on
ESP8266 as that board does not have enough memory to fit certain modules
unless additional configuration is performed.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

tests/run-natmodtests.py

@@ -73,6 +73,7 @@ class __FS:
     return __File()
 vfs.mount(__FS(), '/__remote')
 sys.path.insert(0, '/__remote')
+{import_prelude}
 sys.modules['{}'] = __import__('__injected')
 """
 
@@ -133,6 +134,13 @@ def detect_architecture(target):
 
 
 def run_tests(target_truth, target, args, stats, resolved_arch):
+    global injected_import_hook_code
+
+    prelude = ""
+    if args.begin:
+        prelude = args.begin.read()
+    injected_import_hook_code = injected_import_hook_code.replace("{import_prelude}", prelude)
+
     for test_file in args.files:
         # Find supported test
         test_file_basename = os.path.basename(test_file)
@@ -212,6 +220,13 @@ def main():
     cmd_parser.add_argument(
         "-a", "--arch", choices=AVAILABLE_ARCHS, help="override native architecture of the target"
     )
+    cmd_parser.add_argument(
+        "-b",
+        "--begin",
+        type=argparse.FileType("rt"),
+        default=None,
+        help="prologue python file to execute before module import",
+    )
     cmd_parser.add_argument("files", nargs="*", help="input test files")
     args = cmd_parser.parse_args()