Fix the security update version check message for PyPy (#1040)

Previously if an app was using an older version of PyPy, the buildpack
would show a confusing "Could not find that version" message (even
though the version was found), when it really meant to warn about there
being a newer release available.

It looks like the version check messages were perhaps copied and pasted
from something else, but the message wording not updated at the time.

I've also added tests since there were none for this feature.

Fixes #1004.
Closes @W-7918745@.

bin/steps/python

@@ -8,6 +8,7 @@ PYTHON_VERSION=$(cat runtime.txt)
 VENDORED_PYTHON="${VENDOR_URL}/runtimes/$PYTHON_VERSION.tar.gz"
 
 SECURITY_UPDATE="Python has released a security update! Please consider upgrading to"
+SECURITY_UPDATE_PYPY="The PyPy project has released a security update! Please consider upgrading to"
 
 ONLY_SUPPORTED_2_VERSION="Only the latest version of Python 2 is supported on the platform. Please consider upgrading to"
 
@@ -64,14 +65,14 @@ if curl --output /dev/null --silent --head --fail "$VENDORED_PYTHON"; then
   if [[ "$PYTHON_VERSION" == $PYPY27* ]]; then
     # security update note
     if [ "$PYTHON_VERSION" != "$PYPY_27" ]; then
-      puts-warn "Could not find that Pypy version. Did you mean" "${PYPY_27}?"
+      puts-warn "$SECURITY_UPDATE_PYPY" "$PYPY_27"
       echo "       Learn More: https://devcenter.heroku.com/articles/python-runtimes"
     fi
   fi
   if [[ "$PYTHON_VERSION" == $PYPY36* ]]; then
     # security update note
     if [ "$PYTHON_VERSION" != "$PYPY_36" ]; then
-      puts-warn "Could not find that Pypy version. Did you mean" "${PYPY_36}?"
+      puts-warn "$SECURITY_UPDATE_PYPY" "$PYPY_36"
       echo "       Learn More: https://devcenter.heroku.com/articles/python-runtimes"
     fi
   fi