Use os.TempDir() for test socket path

[ericyan]

Description

This PR replaces runtime.Caller(0)-based socket path construction with os.TempDir() in newTestMigrationContext.

As the path is constructed relative to the source tree, in some CI environments the path might be too long, causing tests to fail due to the fact that Linux enforces a hard limit of 108 characters for Unix socket paths.

This also ensures that each test run no longer shares a hardcoded socket path, reducing the risk of failures if a previous test leaked the socket.

As an additional benefit, it drops the runtime import and the //nolint:dogsled suppression.

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[Copilot]

Pull request overview

Replaces the runtime.Caller(0)-based source-relative socket path with os.TempDir() for the test migration context, avoiding the 108-char Unix socket path limit hit in some CI environments and removing the runtime import and //nolint:dogsled suppression.

Changes:

• Switch ServeSocketFile construction to filepath.Join(os.TempDir(), "gh-ost.sock").
• Drop the runtime import and the associated //nolint:dogsled directive.

Show a summary per file

File	Description
go/logic/test_utils.go	Builds test socket path under the system temp dir instead of relative to the source file.

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