test: set case name from initial filename for a sequence

Port more incremental tests.

test/incremental/assert_function.6.zig

@@ -0,0 +1,9 @@
+pub fn main() void {
+    const a: u32 = 2;
+    const b: ?u32 = a;
+    const c = b.?;
+    if (c != 2) unreachable;
+}
+
+// run
+//