Use labels to enhance records filtering in FA e2e tests (antrea-io#5731)

In this commit, we:

1. Add a label to Perftest Pods before initiating traffic in each subtest to filter records in both the IPFIX collector Pod and the ClickHouse.
2. Remove the --since-time flag used during log retrieval from the IPFIX collector Pod in the e2e test.
3. Stop relying on timestamps for record filtering due to potential time discrepancies between the testbed and Kubernetes nodes, which might hinder the retrieval of desired records.

Signed-off-by: Yun-Tang Hsu <hsuy@vmware.com>