test(frontmatter): Cover spaces between infostring parts

As these characters are specifically called out in the RFC, I felt it
would be important to have a test to cover them.

tests/ui/frontmatter/space-in-infostring.rs

@@ -0,0 +1,9 @@
+--- cargo clippy
+//~^ ERROR: invalid infostring for frontmatter
+---
+
+// infostrings cannot have spaces
+
+#![feature(frontmatter)]
+
+fn main() {}

tests/ui/frontmatter/space-in-infostring.stderr

@@ -0,0 +1,10 @@
+error: invalid infostring for frontmatter
+  --> $DIR/space-in-infostring.rs:1:4
+   |
+LL | --- cargo clippy
+   |    ^^^^^^^^^^^^^
+   |
+   = note: frontmatter infostrings must be a single identifier immediately following the opening
+
+error: aborting due to 1 previous error
+