Fix pylint errors going uncaught

Make check-python-files.sh run pylint on all *.py files (in
directories where they are known to be present), rather than list
files explicitly.

Fix a bug whereby the return status of check-python-files.sh was only
based on the last file passing, i.e. errors in other files were
effectively ignored.

Make check-python-files.sh run pylint unconditionally. Since pylint3
is not critical, make all.sh to skip running check-python-files.sh if
pylint3 is not available.

tests/scripts/all.sh

@@ -1237,6 +1237,9 @@ component_test_zeroize () {
     unset gdb_disable_aslr
 }
 
+support_check_python_files () {
+    type pylint3 >/dev/null 2>/dev/null
+}
 component_check_python_files () {
     msg "Lint: Python scripts"
     record_status tests/scripts/check-python-files.sh