store full host tools command in sites.py variables check.

Description

This changes how we check for host tools availability by setting the variables exported from tests/testutils/sites.py to their whole paths.

This removes the need for tests to manually get the paths and fixes a bug where host tools not in canonical paths would not be found when running tests, but tests would not be skipped.

This merge request, when approved, will close: #936 (closed)

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