Keep JUnit output when test groups are interleaved - #1894

Summary

• Keep one in-memory bucket per test group in -ojunit output so a later visit to the same group no longer truncates earlier tests.
• Snapshot each test's assertion count when the test ends, so interleaved groups still report the right assertions= values.

Problem

JUnitTestOutput buffers a single group and opens cpputest_<group>.xml with mode "w" at every group-end. TestRegistry::endOfGroup treats a name change as the end of a group, so interleaved tests or -s shuffle causes the reporter to throw away the first visit and overwrite the file with only the last fragment.

Problem reported in #1790, #1741, #168, and #1207.

Fix

Keep a linked list of groups (no STL). On each test, append to that group's bucket even if we left it earlier. On group-end, still write the file immediately, but write the full accumulated list. Store the per-test assertion delta at test-end so a rewrite of group A after running group B does not steal B's checks.

Sequential groups produce the same XML shape as before. The JUnit test helper no longer double-runs the last test on a group switch; that only became visible once the reporter stopped wiping the buffer.

Verification

• New tests for interleaved groups: all tests present, failures from the first visit kept, assertion counts not mixed across groups.
• Existing JUnitOutputTest cases still pass, including sequential groups and testFailuresInSeparateGroups.
• CMake/CTest RelWithDebInfo on MSVC: 84/84 passed.