This updates ClearCaseClient to add formal support for dependency

checks. ClearCaseClient.check_dependencies() now determines if

cleartool is installed, and no longer invokes it upon instantiation of

the class.
The ClearCaseClient.host_properties attribute used to be set when the

client was instantiated. Now it's a property that retrieves the

information upon first access, performing a dependency check if needed

(and emitting a warning if one was not already performed).
Unit tests have been updated to include new dependency-related tests.

Since the existing ClearCase tests are designed to use spies to simulate

results, rather than depending on cleartool for test runs, those have

been updated to turn off dependency checks when setting up a client.

Unit tests pass on Python 3.7-3.11.