t/test-lib-functions.sh: implement `test_trace2_data` helper

Introduce a helper function which looks for a specific (category, key,
value) tuple in the output of a trace2 event stream.

We will use this function in a future patch to ensure that the expected
number of objects are reused from an expected number of packs.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -1874,6 +1874,20 @@ test_region () {
 	return 0
 }
 
+# Check that the given data fragment was included as part of the
+# trace2-format trace on stdin.
+#
+#	test_trace2_data <category> <key> <value>
+#
+# For example, to look for trace2_data_intmax("pack-objects", repo,
+# "reused", N) in an invocation of "git pack-objects", run:
+#
+#	GIT_TRACE2_EVENT="$(pwd)/trace.txt" git pack-objects ... &&
+#	test_trace2_data pack-objects reused N <trace2.txt
+test_trace2_data () {
+	grep -e '"category":"'"$1"'","key":"'"$2"'","value":"'"$3"'"'
+}
+
 # Given a GIT_TRACE2_EVENT log over stdin, writes to stdout a list of URLs
 # sent to git-remote-https child processes.
 test_remote_https_urls() {