test: set case name from initial filename for a sequence

Port more incremental tests.

test/incremental/assert_function.15.zig

@@ -0,0 +1,10 @@
+pub fn main() void {
+    assert("hello"[0] == 'h');
+}
+
+pub fn assert(ok: bool) void {
+    if (!ok) unreachable; // assertion failure
+}
+
+// run
+//