Allow to pass or propagate some RUSTFLAGS

[gui1117]

Due to:
https://github.com/dtolnay/trybuild/pull/278/files#diff-36dcce9bb960d87a9efde38a0309b8e3e552a4193594bb2065ced1e749c9b743R41
the latests version doesn't propagate the RUSTFLAGS anymore.

In our usecase we pass deny warnings using the RUSTFLAGS environment variable.

This is very handy to ensure no warnings ever gets emitted from our macros.

#[test]
fn tests() {
	std::env::set_var("RUSTFLAGS", "--deny warnings");
	let t = trybuild::TestCases::new();
	t.compile_fail("tests/*rs");
}

Is there a way to allow giving rustflags maybe with a new TRYBUILD_RUSTFLAGS or something?

If you approve the direction I can make a PR

[gui1117]

I was thinking something like this maybe:

 pub(crate) fn toml() -> toml::Value {
-    let mut rustflags = vec!["--cfg", "trybuild", "--verbose"];
+    let mut rustflags = Vec::<String>::new();

+    // Propagate some rustflags from env:
+    for flag in rustflags::from_encoded(&std::env::var_os("TRYBUILD_RUSTFLAGS").unwrap_or_default()
+
+        ) {
+        match flag {
+            rustflags::Flag::Deny(s) => {
+                rustflags.push("-D".into());
+                rustflags.push(s);
+            },
+            rustflags::Flag::Forbid(s) => {
+                rustflags.push("-D".into());
+                rustflags.push(s);
+            },
+            rustflags::Flag::Allow(s) => {
+                rustflags.push("-A".into());
+                rustflags.push(s);
+            },
+            rustflags::Flag::Warn(s) => {
+                rustflags.push("-W".into());
+                rustflags.push(s);
+            },
+            rustflags::Flag::Cfg { name, value: Some(value) } => {
+                rustflags.push("--cfg".into());
+                rustflags.push(format!("{}=\"{}\"", name, value));
+            },
+            rustflags::Flag::Cfg { name, value: None } => {
+                rustflags.push("--cfg".into());
+                rustflags.push(name.into());
+            },
+            _ => (),
+        }
+    }
+
+    rustflags.extend_from_slice(&["--cfg".into(), "trybuild".into(), "--verbose".into()]);
 
     for &lint in IGNORED_LINTS {
-        rustflags.push("-A");
-        rustflags.push(lint);
+        rustflags.push("-A".into());
+        rustflags.push(lint.into());
     }
 
     toml::Value::try_from(rustflags).unwrap()
 }