Move testing of the prerequisite archive formats

to the separate function 'prereq'. Do not expect any arguments

tests/before

@@ -17,20 +17,22 @@ exec 1> stdout
 exec 2> stderr
 
 PATH=..:../../src:$PATH
-
-if test $# -ne 0; then
-    case $TAR_OPTIONS in
-        --format=*)   FMTOPT=$TAR_OPTIONS;;
-        *)            FMTOPT=`tar --show-defaults`;;
-    esac
-
-    FORMAT=
-    for option
-    do
-        case $FMTOPT in
-            --format=$option*) FORMAT=$option
-	                       break;;
+    
+prereq() {
+    if test $# -ne 0; then
+        case $TAR_OPTIONS in
+            --format=*)   FMTOPT=$TAR_OPTIONS;;
+            *)            FMTOPT=`tar --show-defaults`;;
         esac
-    done
-    test -z "$FORMAT" && exit 77
-fi
+
+        FORMAT=
+        for option
+        do
+            case $FMTOPT in
+                --format=$option*) FORMAT=$option
+       	                           break;;
+            esac
+        done
+        test -z "$FORMAT" && exit 77
+    fi
+}