Make sure fmt-write-bloat doesn't vacuously pass

Previously, the test only checks for the absence of certain panic
symbols, but having no symbols was also a possible albeit vacuous way to
satisfy this assertion.

Fix this by checking we at least observe the `main` symbol which is
always expected to be present.

Author: jieyouxu

tests/run-make/fmt-write-bloat/rmake.rs

@@ -31,5 +31,8 @@ fn main() {
         // otherwise, add them to the list of symbols to deny.
         panic_syms.extend_from_slice(&["panicking", "panic_fmt", "pad_integral", "Display"]);
     }
+    // Make sure we at least have the `main` symbol itself, otherwise even no symbols can trivially
+    // satisfy the "no panic symbol" assertion.
+    assert!(any_symbol_contains(bin_name("main"), &["main"]));
     assert!(!any_symbol_contains(bin_name("main"), &panic_syms));
 }