PF Diff cross tests

[VenelinMartinov]

This change adds the framework code for running Diff cross-tests in PF. It also adds a simple cross-test to show the framework works. I'll add additional coverage in a follow-up PR.

I've also exposed the Stdout output of both TF Plan and Pulumi Preview, so that they can be recorded in the cross-test. This will allow us to use the cross-tests for testing accurate bridge previews.

related to #2297

[VenelinMartinov]

This change is part of the following stack:

• PF Diff cross tests #2578 ◀
  • Use crosstestimpl.T instead of testing.T in all PF crosstests #2584

_{Change managed by git-spice.}

[codecov]

Codecov Report

Attention: Patch coverage is 93.65079% with 8 lines in your changes missing coverage. Please review.

Project coverage is 62.93%. Comparing base (a2113d8) to head (5ec549b).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/pf/tests/internal/cross-tests/diff.go	92.66%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2578      +/-   ##
==========================================
+ Coverage   62.82%   62.93%   +0.10%     
==========================================
  Files         387      388       +1     
  Lines       51717    51837     +120     
==========================================
+ Hits        32493    32625     +132     
+ Misses      17400    17385      -15     
- Partials     1824     1827       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.