test(integration): integrate shared chunk cache tests

[PranjalC100]

Description

This PR integrates the shared chunk cache tests into the integration test suite and updates the test configurations. Specifically, it includes the following changes:

• Adds "shared_chunk_cache" to TEST_PACKAGES_COMMON in tools/integration_tests/improved_run_e2e_tests.sh.
• Refactors bucket references in the shared_chunk_cache tests to use testEnv.cfg.TestBucket instead of setup.TestBucket().
• Updates the flag parsing logic in shared_chunk_cache_test.go to use comma-separated values (strings.Split) instead of spaces (strings.Fields).
• Adds the shared_chunk_cache test block to tools/integration_tests/test_config.yaml to run TestSharedChunkCacheTestSuite with the relevant experimental shared chunk cache flags on GKE. Note that this configuration was removed in an earlier PR (#4836), assuming it to be present in the test_config.yaml.

Link to the issue in case of a bug fix.

b/535670081

Testing details

1. Manual - NA
2. Unit tests - NA
3. Integration tests - This PR specifically updates and configures the integration test suite for the shared chunk cache.

Any backward incompatible change? If so, please explain.

No.

[gemini-code-assist]

Code Review

This pull request integrates the shared_chunk_cache package into the end-to-end integration tests, updates the test configuration, and switches the cleanup logic to use the configured test bucket (testEnv.cfg.TestBucket) instead of the default helper. Additionally, it updates the flag parsing logic to split on commas instead of whitespace. Feedback was provided to make this flag parsing more robust by trimming whitespace and filtering out empty strings to prevent potential test failures from malformed flag strings.

[gemini-code-assist]

Using strings.Split directly can lead to empty or whitespace-only strings in the flags slice if the config contains consecutive, leading, or trailing commas, or spaces around commas (e.g., "--flag1, ,--flag2"). This can cause unexpected test failures. A more robust approach is to trim whitespace from each part and filter out any empty strings.

                                var primaryFlags []string
				for _, f := range strings.Split(flagStr, ",") {
					if flag := strings.TrimSpace(f); flag != "" {
						primaryFlags = append(primaryFlags, flag)
					}
				}
				var secondaryFlags []string
				if len(cfg.SecondaryFlags) > i {
					secondaryFlags = []string{}
					for _, f := range strings.Split(cfg.SecondaryFlags[i], ",") {
						if flag := strings.TrimSpace(f); flag != "" {
							secondaryFlags = append(secondaryFlags, flag)
						}
					}
				}