Unify lit.cfg.py approach (#2249)

On #2224 @zygoloid pointed out we needed --implicit-check-not to ensure we were correctly matching output. This is the standard way we're writing explorer tests, so I was looking at unifying our lit approaches.

This is one take on it, making more use of substitutions to bring various testing into alignment, as well as symlinks to avoid config skew (maybe I'll eventually figure out a better solution than symlinks).

Makes a couple small fixes in explorer to remove end-of-line whitespace on output.

explorer/testdata/interface/fail_external_impl_self.carbon

@@ -2,10 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{not} %{explorer} %s 2>&1 | \
-// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
-// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
-// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// RUN: %{not} %{explorer} %s 2>&1 | %{FileCheck-strict} %s
+// RUN: %{not} %{explorer-trace} %s 2>&1 | %{FileCheck-allow-unmatched} %s
 // AUTOUPDATE: %{explorer} %s
 
 package ExplorerTest api;
@@ -20,7 +18,7 @@ class Point {
   var y: i32;
 }
 
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_external_impl_self.carbon:[[@LINE+1]]: could not resolve 'Self'
+// CHECK:COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_external_impl_self.carbon:[[@LINE+1]]: could not resolve 'Self'
 external impl Self as Vector {
   fn Add[me: Point](b: Point) -> Point {
     return {.x = me.x + b.x, .y = me.y + b.y};