pyln-testing: fail typecheck gracefully on integer.

We say the offer amount arg can be either an `msat_or_any` or a `currency` string, but technically the order of evaluation is undefined, so it can check the other way, and it should not crash:

```
>       offer = l3.rpc.offer(1000, 'test_pay_blindedpath_nodeaddr')

tests/test_pay.py:5692: 
...
checker = <TypeChecker types={'array', 'bip340sig', 'boolean', 'currency', 'feerate', 'hash', 'hex', 'integer', 'msat', 'msat_or..._all', 'secret', 'short_channel_id', 'short_channel_id_dir', 'signature', 'string', 'txid', 'u16', 'u32', 'u64', 'u8'}>
instance = 1000

    def is_currency(checker, instance):
        """currency including currency code"""
        pattern = re.compile(r'^\d+(\.\d+)?[A-Z][A-Z][A-Z]$')
>       if pattern.match(instance):
E       TypeError: expected string or bytes-like object

```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

contrib/pyln-testing/pyln/testing/fixtures.py

@@ -403,7 +403,7 @@ def is_sat_or_all(checker, instance):
     def is_currency(checker, instance):
         """currency including currency code"""
         pattern = re.compile(r'^\d+(\.\d+)?[A-Z][A-Z][A-Z]$')
-        if pattern.match(instance):
+        if checker.is_type(instance, "string") and pattern.match(instance):
             return True
         return False