Fix concurrent map access in LogMonitor causing flaky test crashes

[prafgup]

Issue #, if available:

Failure - https://github.com/awslabs/soci-snapshotter/actions/runs/22316452115/job/64568052875

=== RUN   TestLazyPullNoIndexDigest
=== RUN   TestLazyPullNoIndexDigest/normal
fatal error: concurrent map iteration and map write

goroutine 5519 [running]:
internal/runtime/maps.fatal({0xe03530?, 0xad?})
	/codebuild/output/src2529771585/src/actions-runner/_work/_tool/go/1.25.7/arm64/src/runtime/panic.go:1046 +0x20
internal/runtime/maps.(*Iter).Next(0x40000500b0?)
	/codebuild/output/src2529771585/src/actions-runner/_work/_tool/go/1.25.7/arm64/src/internal/runtime/maps/table.go:792 +0x98
github.com/awslabs/soci-snapshotter/util/testutil.(*LogMonitor).scanLog(0x400007e230, {0xf22720?, 0x40009b02c0?})
	/codebuild/output/src2529771585/src/actions-runner/_work/soci-snapshotter/soci-snapshotter/util/testutil/shell.go:146 +0xe0
created by github.com/awslabs/soci-snapshotter/util/testutil.NewLogMonitor in goroutine 5525
	/codebuild/output/src2529771585/src/actions-runner/_work/soci-snapshotter/soci-snapshotter/util/testutil/shell.go:119 +0x1dc

Description of changes:

• LogMonitor.monitorFuncs was a plain map[string]func(string) accessed concurrently by scanLog goroutines and test code calling Add/Remove, causing fatal error: concurrent map iteration and map write.
• Replaced with sync.Map

Testing performed:

Ran integration tests -

• TestLazyPullNoIndexDigest

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sondavidb]

#1778 should also fix this FWIW.

[sondavidb]

#1778 is merged so I think this is not needed anymore. Feel free to reopen if you feel otherwise 🙂