Fix unit test failure of semaphore and cluster

[benja-wu]

1. Changing to use golang.org/x/sync.sem instead of implementing it by ourselves with chan struct{}.
2. Had run make test successfully in MacBook env.

49.474833ms
/abc/com/merchant/*/other/30/details
--- PASS: TestURLClusterAnalyzer (0.05s)
PASS
ok  	github.com/megaease/easegress/pkg/util/urlclusteranalyzer	(cached)
=== RUN   TestURLRULEMatch
--- PASS: TestURLRULEMatch (0.00s)
=== RUN   TestURLRegxMatch
--- PASS: TestURLRegxMatch (0.00s)
=== RUN   TestURLExactMatch
--- PASS: TestURLExactMatch (0.00s)
=== RUN   TestURLExactNotMatch
--- PASS: TestURLExactNotMatch (0.00s)
=== RUN   TestURLPrefixNotMatch
--- PASS: TestURLPrefixNotMatch (0.00s)
=== RUN   TestURLRULENoMatchMethod
--- PASS: TestURLRULENoMatchMethod (0.00s)
=== RUN   TestURLRULENoMatchURL
--- PASS: TestURLRULENoMatchURL (0.00s)
PASS
ok  	github.com/megaease/easegress/pkg/util/urlrule	(cached)
?   	github.com/megaease/easegress/pkg/util/yamltool	[no test files]
?   	github.com/megaease/easegress/pkg/v	[no test files]
?   	github.com/megaease/easegress/pkg/version	[no test files]
√ easegress $ 

[benja-wu]

Also, this PR aims to solve the cluster test case failure

2021-07-08T02:51:50.758Z	ERROR	cluster/cluster.go:679	register cluster name test-cluster failed: create client failed: couldn't open sink "/tmp/eg-test/test-member-001/log/etcd_client.log": open /tmp/eg-test/test-member-001/log/etcd_client.log: no such file or directory
panic: register cluster name test-cluster failed: create client failed: couldn't open sink "/tmp/eg-test/test-member-001/log/etcd_client.log": open /tmp/eg-test/test-member-001/log/etcd_client.log: no such file or directory

1. Changing /tmp directory to os.TempDir(). (Recommend by @zhao-kun)
2. Adding retry logic into the cluster.client in the test case.