Fix erroneous agent expression test

Testing of the expression rewrite revealed a buglet in ax.exp.  One
test does:

    gdb_test "maint agent (unsigned char)1L" ".*ext 8.*"

However, zero extension is not actually needed in this case -- a
simple "const8 1" is also correct here.

This patch changes the test to look for a push of any width of the
constant 1.

gdb/testsuite/ChangeLog
2021-01-09  Tom Tromey  <tom@tromey.com>

	* gdb.trace/ax.exp: Do not require an "ext".