Resolve the TESTS list with a union merge

Every new test appends to the same last line of tests/Makefile.am's TESTS list,
so any two branches in flight collide there. A union merge keeps both entries
and automake does not care about the order.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

.gitattributes

@@ -6,3 +6,8 @@
 # bash die on $'\r' on every line of them.
 *.test text eol=lf
 *.sh text eol=lf
+
+# Every new test appends to the TESTS list, so two branches in flight always
+# collide on its last line. A union merge keeps both entries; automake does not
+# care about order.
+tests/Makefile.am merge=union