server/join: add unit and integration test coverage

[ljluestc]

Add server/join/join_test.go with white-box unit tests for the two previously untested code paths:

• TestIsDataExist: covers all three branches of the isDataExist helper (non-existent directory, empty directory, directory with content).
• TestPrepareJoinClusterEmptyJoin: verifies the no-op path when Join is empty (new initial leader).
• TestPrepareJoinClusterJoinSelf: verifies the 'join self is forbidden' guard.
• TestPrepareJoinClusterWithExistingDataDir: verifies that when the member data directory already has content the function sets InitialCluster to "" and InitialClusterState to "existing" without dialling the join address.

Also add TestJoinWithAbnormalExistingMember to the existing integration test suite (tests/server/join/join_test.go). It exercises the error path where the member list contains an entry with an empty Name (a node that completed MemberAdd but never started) whose peer URLs differ from those of the new PD attempting to join.

Issue Number: ref #2288

What problem does this PR solve?

Issue Number: Close #xxx

What is changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP APIs changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

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Hi @ljluestc. Thanks for your PR.

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