Use "require" for Python tests

This changes various tests to use "require" for the Python feature.

gdb/testsuite/gdb.python/py-pp-integral.exp

@@ -15,13 +15,12 @@
 
 standard_testfile
 
+require !skip_python_tests
+
 if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
     return -1
 }
 
-# Skip all tests if Python scripting is not enabled.
-if { [skip_python_tests] } { continue }
-
 if ![runto tick_tock] {
     return -1
 }