Fix evaluation of conditional expressions at negative cycle points

[hjoliver]

Integer offsets [-P2] or bigger aren't getting replaced with Python code in prerequisite conditional expressions because they generate negative cycle points when evaluated at initial cycle point 1. (There's no problem with [-P1] - that generates a cycle point of 0).

Close #6588

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

TODO: consider the preceding comment - might need a slightly more sophisticated fix?

[hjoliver]

(Tests all pass, but maybe the commented issue only tested the other word boundary for task name sub-strings)

[MetRonnie]

The test added in #4426 is no longer valid since universal ID was implemented.

This is needed to make the test correct

diff --git a/cylc/flow/prerequisite.py b/cylc/flow/prerequisite.py
index 717276affc..0ad3ef7eee 100644
--- a/cylc/flow/prerequisite.py
+++ b/cylc/flow/prerequisite.py
@@ -193,14 +193,13 @@ class Prerequisite:
         Examples:
             # GH #3644 construct conditional expression when one task name
-            # is a substring of another: foo | xfoo => bar.
-            # Add 'foo' to the 'satisfied' dict before 'xfoo'.
+            # is a substring of another: 11/foo | 1/foo => bar.
             >>> preq = Prerequisite(1)
             >>> preq[(1, 'foo', 'succeeded')] = False
-            >>> preq[(1, 'xfoo', 'succeeded')] = False
-            >>> preq.set_conditional_expr("1/foo succeeded|1/xfoo succeeded")
+            >>> preq[(11, 'foo', 'succeeded')] = False
+            >>> preq.set_conditional_expr("11/foo succeeded|1/foo succeeded")
             >>> expr = preq.conditional_expression
             >>> expr.split('|')  # doctest: +NORMALIZE_WHITESPACE
-            ['bool(self._satisfied[("1", "foo", "succeeded")])',
-            'bool(self._satisfied[("1", "xfoo", "succeeded")])']
+            ['bool(self._satisfied[("11", "foo", "succeeded")])',
+            'bool(self._satisfied[("1", "foo", "succeeded")])']