Adapt check files

Author: odersky

tests/neg/extension-cannot-have-type.check

@@ -1,6 +1,4 @@
--- [E151] Syntax Error: tests/neg/extension-cannot-have-type.scala:3:10 ------------------------------------------------
+-- Error: tests/neg/extension-cannot-have-type.scala:3:10 --------------------------------------------------------------
 3 |    def f[U](u: U): T = ??? // error : extension method cannot have type params
   |          ^
-  |          Extension method cannot have type parameters since some were already given previously
-
-longer explanation available when compiling with `-explain`
+  |          extension method cannot have type parameters since some were already given previously

tests/neg/extension-method-not-allowed.check

@@ -1,6 +1,4 @@
--- [E150] Syntax Error: tests/neg/extension-method-not-allowed.scala:3:8 -----------------------------------------------
+-- Error: tests/neg/extension-method-not-allowed.scala:3:8 -------------------------------------------------------------
 3 |    def (c: T).f: T = ??? // error : No extension method allowed here
   |    ^^^^^^^^^^^^^^^^^^^^^
-  |    No extension method allowed here, since collective parameters are given
-
-longer explanation available when compiling with `-explain`
+  |    no extension method allowed here since leading parameter was already given

tests/neg/extensions-can-only-have-defs.check

@@ -1,6 +1,4 @@
--- [E152] Syntax Error: tests/neg/extensions-can-only-have-defs.scala:3:8 ----------------------------------------------
+-- Error: tests/neg/extensions-can-only-have-defs.scala:3:8 ------------------------------------------------------------
 3 |    val v: T = ??? // error : extensions can only have defs
   |    ^^^^^^^^^^^^^^
-  |    Only methods allowed here, since collective parameters are given
-
-longer explanation available when compiling with `-explain`
+  |    extension clause can only define methods

tests/neg/i6900.scala

@@ -4,12 +4,12 @@ object Test2 {
   extension on [A](a: A):
     def foo[C]: C => A = _ => a   // error: extension method cannot have type parameters
 
-  1.foo.foo                      // error: foo is undefined
+  1.foo.foo
 
   // ... but have to pass 2 parameters
-  1.foo.foo[Any => Int, String]                  // error: foo is undefined
-  1.foo[Int, String].foo                         // error: foo is undefined
-  1.foo[Int, String].foo[String => Int, String]  // error: foo is undefined
+  1.foo.foo[Any => Int, String]
+  1.foo[Int, String].foo
+  1.foo[Int, String].foo[String => Int, String]
 
 }