Add an env var to force meson to print a backtrace

This is really useful when debugging test failures. Without a stack
trace, you have to grep the source code for the error message.

Also set this in run_tests.py.

mesonbuild/mesonmain.py

@@ -312,7 +312,11 @@ def run(mainfile, args):
             else:
                 mlog.log(mlog.red('\nMeson encountered an error:'))
             mlog.log(e)
+            if os.environ.get('MESON_FORCE_BACKTRACE'):
+                raise
         else:
+            if os.environ.get('MESON_FORCE_BACKTRACE'):
+                raise
             traceback.print_exc()
         return 1
     return 0