Go 1.26: Test modernization — T.Chdir() adoption

[spboyer]

Parent Issue

Part of #7081 — Go 1.26 Modernization

Description

Adopt T.Chdir() (Go 1.24) in tests that manually change working directories.

Pattern

// Before
oldDir, _ := os.Getwd()
os.Chdir(tempDir)
defer os.Chdir(oldDir)

// After
t.Chdir(tempDir) // auto-restored when test ends

Benefits

• Safer — directory always restored even on panic
• No forgotten defers
• Cleaner test code