Make load handle absolute paths too

Add 2 simple tests on this.

libexec/bats-exec-test

@@ -30,13 +30,20 @@ BATS_TEST_DIRNAME="$(dirname "$BATS_TEST_FILENAME")"
 BATS_TEST_NAMES=()
 
 load() {
-  local filename="$BATS_TEST_DIRNAME/$1.bash"
+  local inc="$1"
+  local filename
+
+  if [[ "${inc}" == "/"* ]] ; then
+    filename="${inc}"
+  else
+    filename="$BATS_TEST_DIRNAME/${inc}.bash"
+  fi
   [ -f "$filename" ] || {
     echo "bats: $filename does not exist" >&2
     exit 1
   }
 
-  source "$BATS_TEST_DIRNAME/$1.bash"
+  source "${filename}"
 }
 
 run() {