Avoid missing main error in tests

rust-lang/rust#64290 changed that lint output causing test error, however there is no point in keeping this lint stderr

phf_macros/tests/compile-fail-unicase/equivalent-keys.rs

@@ -5,3 +5,5 @@ static MAP: phf::Map<UniCase<&'static str>, isize> = phf_map!( //~ ERROR duplica
     UniCase("FOO") => 42, //~ NOTE one occurrence here
     UniCase("foo") => 42, //~ NOTE one occurrence here
 );
+
+fn main() {}

phf_macros/tests/compile-fail-unicase/equivalent-keys.stderr

@@ -3,9 +3,3 @@ error: unsupported key expression
   |
 5 |     UniCase("FOO") => 42, //~ NOTE one occurrence here
   |     ^^^^^^^^^^^^^^
-
-error[E0601]: `main` function not found in crate `$CRATE`
-  |
-  = note: consider adding a `main` function to `$DIR/tests/compile-fail-unicase/equivalent-keys.rs`
-
-For more information about this error, try `rustc --explain E0601`.

phf_macros/tests/compile-fail/bad-syntax.rs

@@ -5,3 +5,5 @@ static MAP: phf::Map<u32, u32> = phf_map! {
     => //~ ERROR expected identifier
     ()
 };
+
+fn main() {}

phf_macros/tests/compile-fail/bad-syntax.stderr

@@ -3,9 +3,3 @@ error: expected identifier
   |
 5 |     => //~ ERROR expected identifier
   |     ^^
-
-error[E0601]: `main` function not found in crate `$CRATE`
-  |
-  = note: consider adding a `main` function to `$DIR/tests/compile-fail/bad-syntax.rs`
-
-For more information about this error, try `rustc --explain E0601`.