test: only validate MTU if Eth0MTU is present

[jackfrancis]

Reason for Change:

This PR tweaks the E2E test implementation so that /sys/class/net/eth0/mtu is not validated unless the LinuxProfile.Eth0MTU property is present (e.g., kubenet).

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[codecov]

Codecov Report

Merging #4434 (ba0ead4) into master (60828c7) will increase coverage by 0.00%.
The diff coverage is 71.89%.

@@           Coverage Diff            @@
##           master    #4434    +/-   ##
========================================
  Coverage   72.04%   72.05%            
========================================
  Files         141      141            
  Lines       21631    21764   +133     
========================================
+ Hits        15584    15681    +97     
- Misses       5096     5131    +35     
- Partials      951      952     +1     

Impacted Files	Coverage Δ
cmd/rotate_certs.go	11.03% <0.00%> (ø)
cmd/upgrade.go	35.92% <0.00%> (ø)
pkg/api/common/versions.go	96.37% <ø> (ø)
pkg/api/types.go	92.85% <ø> (ø)
pkg/api/vlabs/types.go	73.04% <ø> (ø)
pkg/engine/templates_generated.go	43.31% <ø> (ø)
pkg/engine/template_generator.go	66.37% <11.53%> (-2.17%)	⬇️
cmd/get_logs.go	49.57% <30.43%> (-1.79%)	⬇️
pkg/api/addons.go	98.01% <100.00%> (ø)
pkg/api/converterfromapi.go	95.71% <100.00%> (+<0.01%)	⬆️
... and 4 more

---

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[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: haofan-ms, jackfrancis

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