tests: fix variable name regular expression in _genrestoreenv()

Update the code to correctly anchor the expression on the end of the name, to
require that the entire name match this expression.  It was already anchored at
the start by using re.match(), but this does not anchor it at the end.

tests/run-tests.py

@@ -905,7 +905,7 b' class Test(unittest.TestCase):'
 
         # Only restore environment variable names that the shell allows
         # us to export.
-        name_regex = re.compile('[a-zA-Z][a-zA-Z0-9_]*')
+        name_regex = re.compile('^[a-zA-Z][a-zA-Z0-9_]*$')
 
         with open(scriptpath, 'w') as envf:
             for name, value in os.environ.items():