chore: make {merge,test}-pr.sh scripts accept hash-preceded PR number

Previously one needed to provide only digits, (e.g. `123`), otherwise
scripts would fail to recognise supplied argument as an issue number.

Github almost everywhere precedes PR numbers with hash sign, so it might
be easier/faster to copypaste PR number with along with preceding hash.

With the change scripts will ignore the first preceding hash in the
supplied PR number.

merge-pr.sh

@@ -31,7 +31,7 @@
 
 set -e -o pipefail
 
-readonly PR=$1
+readonly PR="${1###}"
 
 # make sure to add newlines to the message, otherwise merge message
 # will not look well

test-pr.sh

@@ -30,7 +30,7 @@
 
 set -e -o pipefail
 
-readonly PR=$1
+readonly PR="${1###}"
 
 # make sure to add newlines to the message, otherwise merge message
 # will not look well