CI: Eliminating dependency on clone_tests_repo()

As part of archiving the tests repo, we are eliminating the dependency on
`clone_tests_repo()`. The scripts using the function is as follows:

- `ci/install_rust.sh`.
- `ci/setup.sh`
- `ci/lib.sh`

This commit removes or replaces the files, and makes an adjustment accordingly.

Signed-off-by: Hyounggyu Choi <Hyounggyu.Choi@ibm.com>

tools/osbuilder/rootfs-builder/rootfs.sh

@@ -651,7 +651,7 @@ EOF
 				detect_rust_version || \
 					die "Could not detect the required rust version for AGENT_VERSION='${AGENT_VERSION:-main}'."
 			fi
-			bash ${script_dir}/../../../ci/install_rust.sh ${RUST_VERSION}
+			bash ${script_dir}/../../../tests/install_rust.sh ${RUST_VERSION}
 		fi
 		test -r "${HOME}/.cargo/env" && source "${HOME}/.cargo/env"