Moved `bashcomp-parse-line' to the test file, since it's only used there.
Put test cases for `bashcomp-add-suffix' back.
Renamed tests' tag names.
Added comment about support for partial-completion.
Removed dependency on "sz-testutils.el".

The only macro used from there was replaced by a simpler
implementation in "bashcomp-tests.el".
Made test-case `bashcomp-test-execute-one-completion' more robust.