perf-test: optionally print description for each group of tests

Output from tests is indented slightly in the same style as the
correctness tests.

performance-test/perf-test-lib.sh

@@ -72,7 +72,7 @@ print_header () {
 }
 
 time_run () {
-    printf "%-22s" "$1"
+    printf "  %-22s" "$1"
     if test "$verbose" != "t"; then exec 4>test.output 3>&4; fi
     if ! eval >&3 "/usr/bin/time -f '%e\t%U\t%S\t%M\t%I/%O' $2" ; then
 	test_failure=$(($test_failure + 1))
@@ -92,3 +92,6 @@ time_done () {
 
 cd -P "$test" || error "Cannot setup test environment"
 test_failure=0
+
+echo
+echo $(basename "$0"): "Testing ${test_description:-notmuch performance}"