tests: Skip tests needing machine module if (u)machine doesn't exist.

tests/basics/subclass_native_call.py

@@ -3,14 +3,14 @@
 # For this test we need a native class that can be subclassed (has make_new)
 # and is callable (has call).  The only one available is machine.Signal, which
 # in turns needs PinBase.
+try:
     try:
         import umachine as machine
     except ImportError:
         import machine
-try:
     machine.PinBase
     machine.Signal
-except AttributeError:
+except:
     print("SKIP")
     raise SystemExit
 

tests/extmod/machine_pinbase.py

+try:
     try:
         import umachine as machine
     except ImportError:
         import machine
-try:
     machine.PinBase
-except AttributeError:
+except:
     print("SKIP")
     raise SystemExit
 

tests/extmod/machine_pulse.py

+try:
     try:
         import umachine as machine
     except ImportError:
         import machine
-try:
     machine.PinBase
     machine.time_pulse_us
-except AttributeError:
+except:
     print("SKIP")
     raise SystemExit
 

tests/extmod/machine_signal.py

 # test machine.Signal class
 
+try:
     try:
         import umachine as machine
     except ImportError:
         import machine
-try:
     machine.PinBase
     machine.Signal
-except AttributeError:
+except:
     print("SKIP")
     raise SystemExit