test(lexer): Ensure tabs are accepted for horizontal whitespace

Author: epage

tests/ui/frontmatter/frontmatter-whitespace-3.rs

@@ -1,7 +1,7 @@
 
 
----cargo   
----   
+---cargo   	
+---   	
 
 // please note the whitespace characters after the first four lines.
 // This ensures that we accept whitespaces before the frontmatter, after
@@ -10,6 +10,7 @@
 //@ check-pass
 // ignore-tidy-end-whitespace
 // ignore-tidy-leading-newlines
+// ignore-tidy-tab
 
 #![feature(frontmatter)]
 

tests/ui/frontmatter/frontmatter-whitespace-4.rs

@@ -1,7 +1,8 @@
---- cargo
+--- 	cargo
 ---
 
 //@ check-pass
+// ignore-tidy-tab
 // A frontmatter infostring can have leading whitespace.
 
 #![feature(frontmatter)]