Merge #16921: tests: Add information on how to add Vulture suppressions

72a18a73af tests: Add information on how to add Vulture suppressions (practicalswift)

Pull request description:

  Add information on how to add `vulture` suppressions.

  As requested by MarcoFalke in https://github.com/bitcoin/bitcoin/issues/16906#issuecomment-533264107 -- your wish is my command! :)

ACKs for top commit:
  fanquake:
    ACK 72a18a73af - similar sort of message as in [lint-spelling.sh](https://github.com/bitcoin/bitcoin/blob/master/test/lint/lint-spelling.sh).

Tree-SHA512: b347f8cea33d4b0ba987a972979b0ac3423938084fea923a2c457a8081bc839a94ad818689d147477104b9197dc35be413f76a96026cd1507b4411d7513e3464

test/lint/lint-python-dead-code.sh

@@ -13,7 +13,11 @@ if ! command -v vulture > /dev/null; then
     exit 0
 fi
 
-vulture \
+VULTURE_SUPPRESSIONS=$(dirname "${BASH_SOURCE[0]}")/lint-python-dead-code-whitelist
+if ! vulture \
     --min-confidence 60 \
     $(git rev-parse --show-toplevel) \
-    $(dirname "${BASH_SOURCE[0]}")/lint-python-dead-code-whitelist
+    "${VULTURE_SUPPRESSIONS}"; then
+    echo "False positives? Suppressions can be added to ${VULTURE_SUPPRESSIONS}"
+    exit 1
+fi