Fix flaky issue validation tests

Match complete issue-list entries when checking validation messages
instead of searching for issue ID substrings.

A randomly generated commit SHA may contain the same substring and cause
false test failures or successes.

The flaky assertions are fixed in:

* testSuggestedMatchingSingleNonExisting
* testSuggestedMatchingMultipleOneNonExsting
* testSuggestedMatchingMultipleSomeNonExsting
* testSuggestedMatchingMultipleIOExceptionIsNonExsting

The other validation tests are updated to use the shared bug ID and
commit-message constants, consolidating the test data introduced by this
fix without changing their behavior.

Bug: Issue 564285978
Change-Id: I460dea02c2bd60cf8e02e0ff448822789892c651