Replace manual context creation with t.Context() which is automatically
cancelled when the test completes.

Go 1.24 modernization pattern.

Assisted-By: cagent
Signed-off-by: David Gageot <david.gageot@docker.com>

Use t.Setenv() which automatically restores the original value when
the test completes, eliminating the need for manual cleanup.

Go 1.18 modernization pattern.

Assisted-By: cagent
Signed-off-by: David Gageot <david.gageot@docker.com>

Use t.TempDir() which automatically cleans up the temporary directory
when the test completes, eliminating the need for manual cleanup.

Go 1.14 modernization pattern.

Assisted-By: cagent
Signed-off-by: David Gageot <david.gageot@docker.com>

Add linter rules to prevent usage of context.Background() and
context.TODO() in test files - t.Context() should be used instead.

The rules only apply to *_test.go files, not production code.

Note: os.Setenv is not covered by forbidigo due to a limitation where
it only catches calls when the return value is assigned. However,
errcheck will flag unchecked os.Setenv calls.

Assisted-By: cagent
Signed-off-by: David Gageot <david.gageot@docker.com>