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/basic_syntax/fail_var_named_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;
@@ -14,7 +12,7 @@ fn Main() -> i32 {
   // Error: can't use keyword `Self` as the name of a variable.
   // TODO: Current error message is unclear, better would be to say
   // something like: unexpected `Self`, expecting identifier
-  // CHECK: SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_var_named_self.carbon:[[@LINE+1]]: syntax error, unexpected COLON, expecting SLASH or binary *
+  // CHECK:SYNTAX ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_var_named_self.carbon:[[@LINE+1]]: syntax error, unexpected COLON, expecting SLASH or binary *
   var Self : i32 = 0;
   return Self;
 }