icasefs: refactor "test-casecollision-merge.t" to reuse in succeeding fixes
This patch refactors "test-casecollision-merge.t" to increase
reusability in succeeding patches.
It is confirmed that changed test also can detect issue3370.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present