Rename travis_fold function to foldable

We are no longer using Travis. As such, it is confusing to anyone
reading the code to see a function prefixed 'travis_' in GitHub actions
code.
This change renames the travis_fold function to 'foldable', as a first
step towards eliminating such confusing constructs from the repository
where possible.

Signed-off-by: Daniel Müller <deso@posteo.net>

.github/actions/build-selftests/helpers.sh

@@ -1,7 +1,7 @@
 # $1 - start or end
 # $2 - fold identifier, no spaces
 # $3 - fold section description
-travis_fold() {
+foldable() {
   local YELLOW='\033[1;33m'
   local NOCOLOR='\033[0m'
   if [ -z ${GITHUB_WORKFLOW+x} ]; then