Default values are only used if the arguments are not specified. In your case you did specify the arguments - both were supplied, with a value of NULL. (Yes, in this case NULL is considered a real value :-). Try:
EXEC TEST()
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Addendum: The default values for procedure parameters are certainly buried in a system table somewhere (see the SYS.ALL_ARGUMENTS view), but getting the default value out of the view involves extracting text from a LONG field, and is probably going to prove to be more painful than it's worth. The easy way is to add some code to the procedure:
CREATE OR REPLACE PROCEDURE TEST(X IN VARCHAR2 DEFAULT 'P',
Y IN NUMBER DEFAULT 1)
AS
varX VARCHAR2(32767) := NVL(X, 'P');
varY NUMBER := NVL(Y, 1);
BEGIN
DBMS_OUTPUT.PUT_LINE('X=' || varX || ' -- ' || 'Y=' || varY);
END TEST;