examples: Poll for expected server logs in test

[arjan-bal]

Fixes: #8046

Problem

The test is flaky due to a race condition in the driver script. Currently, the script waits for the client to exit and immediately validates the server output:

grpc-go/examples/examples_test.sh

Lines 195 to 219 in 49e224f

	if ! timeout 20 go run ${example}/*client/*.go $client_args &> $CLIENT_LOG; then
	fail "client failed to communicate with server
	got server log:
	$(cat $SERVER_LOG)
	got client log:
	$(cat $CLIENT_LOG)
	"
	else
	pass "client successfully communicated with server"
	fi
	# Check server log for expected output if expecting an
	# output
	if [ -n "${EXPECTED_SERVER_OUTPUT[$example]}" ]; then
	if ! grep -q "${EXPECTED_SERVER_OUTPUT[$example]}" $SERVER_LOG; then
	fail "server log missing output: ${EXPECTED_SERVER_OUTPUT[$example]}
	got server log:
	$(cat $SERVER_LOG)
	got client log:
	$(cat $CLIENT_LOG)
	"
	else
	pass "server log contains expected output: ${EXPECTED_SERVER_OUTPUT[$example]}"
	fi
	fi

In the graceful shutdown example, the expected log is written only after the shutdown is complete:

grpc-go/examples/features/gracefulstop/server/main.go

Line 99 in 49e224f

log.Println("Server stopped gracefully.")

If the test driver validates the server output after the client stream closes but before the server logs the expected message, the test fails.

Fix

This PR updates the test driver to poll for the expected log in the server output, instead of checking it only once. Additionally, the shutdown of the test server is blocked until either the server logs a failure or a success message.

Validation

To reproduce the failure, I added a 1s sleep just before the line log.Println("Server stopped gracefully.") in the server code. I verified that the test no longer fails with this fix, even with the artificial delay.

RELEASE NOTES: N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.28%. Comparing base (3e475d7) to head (6840670).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8871      +/-   ##
==========================================
+ Coverage   82.47%   83.28%   +0.80%     
==========================================
  Files         414      414              
  Lines       32751    32751              
==========================================
+ Hits        27011    27276     +265     
+ Misses       4089     4071      -18     
+ Partials     1651     1404     -247     

see 30 files with indirect coverage changes

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