[Test] Fix tests test_pcluster_configure and test_build_image

[gmarciani]

Description of changes

Fix the following integration tests:

1. test_pcluster_configure: the test has always assumed that the free tier only returns t*.micro instance types. However, new instance types have been recently dded to the free tier by AWS, so the test started failing intermittently because we cannot guarantee the t*.micro are returned as first element by EC2. The fix consists in making the test more flexible and succeed as long as the proposed instance type belong to the free tier, despite being a t*micro type.
2. test_build_image: the test started failing consistently on Rocky9 because the test was using our first stage AMI as parent image, which is Rocky9.5 that does not gets updates, ultimately leading to build failing when the cookbook attempts top install OS packages due to kernel pinning. With this change we are now validating the build image on the latest Rocky9 image (as of today Rocky9.6) also setting UpdateOsPackages to true.
   Notice that Rocky 9.5 reached end os support on June 4th, 2025; so it's even better to start tracking the build-image on the new version, which is Rocky 9.6..

Tests

• [SUCCESS] test_pcluster_configure
• [SUCCESS] test_build_image on rocky9

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