Provide local paths for file tests. (#2830)

The intent of this change is that instead of paths looking like `explorer/testdata/foo/bar.carbon` (repo-relative), they're now just `bar.carbon` (local). The consequence is that paths should be a bit more durable in various environments, and just paths should be shorter and easier to read.

The explorer's prelude is an exception to this since it comes from data, rather than being the test target. Due to the change in approaches, it needs the regex again.

Uses #2829

explorer/testdata/pointer/fail_delete_uninit.carbon

@@ -8,7 +8,7 @@ package ExplorerTest api;
 
 fn Main() -> i32 {
   var p: i32*;
-  // CHECK:STDERR: RUNTIME ERROR: explorer/testdata/pointer/fail_delete_uninit.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<p>>
+  // CHECK:STDERR: RUNTIME ERROR: fail_delete_uninit.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<p>>
   heap.Delete(p);
   return 0;
 }

explorer/testdata/pointer/fail_intrinsic_delete_no_args.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: COMPILATION ERROR: explorer/testdata/pointer/fail_intrinsic_delete_no_args.carbon:[[@LINE+1]]: __intrinsic_delete takes 1 argument
+  // CHECK:STDERR: COMPILATION ERROR: fail_intrinsic_delete_no_args.carbon:[[@LINE+1]]: __intrinsic_delete takes 1 argument
   __intrinsic_delete();
   return 0;
 }

explorer/testdata/pointer/fail_intrinsic_delete_type.carbon

@@ -8,7 +8,7 @@ package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32;
-  // CHECK:STDERR: COMPILATION ERROR: explorer/testdata/pointer/fail_intrinsic_delete_type.carbon:[[@LINE+3]]: type error in *
+  // CHECK:STDERR: COMPILATION ERROR: fail_intrinsic_delete_type.carbon:[[@LINE+3]]: type error in *
   // CHECK:STDERR: expected a pointer type
   // CHECK:STDERR: actual: i32
   __intrinsic_delete(x);

explorer/testdata/pointer/fail_intrinsic_new_no_args.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: COMPILATION ERROR: explorer/testdata/pointer/fail_intrinsic_new_no_args.carbon:[[@LINE+1]]: __intrinsic_new takes 1 argument
+  // CHECK:STDERR: COMPILATION ERROR: fail_intrinsic_new_no_args.carbon:[[@LINE+1]]: __intrinsic_new takes 1 argument
   __intrinsic_new();
   return 0;
 }

explorer/testdata/pointer/fail_invalid_ptr_conversion1.carbon

@@ -11,7 +11,7 @@ class A {}
 fn Main() -> i32 {
   var a: A = {};
   var b: A* = &a;
-  // CHECK:STDERR: COMPILATION ERROR: explorer/testdata/pointer/fail_invalid_ptr_conversion1.carbon:[[@LINE+1]]: type error in initializer of variable: 'class A*' is not implicitly convertible to 'i32'
+  // CHECK:STDERR: COMPILATION ERROR: fail_invalid_ptr_conversion1.carbon:[[@LINE+1]]: type error in initializer of variable: 'class A*' is not implicitly convertible to 'i32'
   var c: i32 = b;
   return 1;
 }

explorer/testdata/pointer/fail_invalid_ptr_conversion2.carbon

@@ -11,7 +11,7 @@ class A {}
 fn Main() -> i32 {
   var a: i32 = 0;
   var b: i32* = &a;
-  // CHECK:STDERR: COMPILATION ERROR: explorer/testdata/pointer/fail_invalid_ptr_conversion2.carbon:[[@LINE+1]]: type error in initializer of variable: 'i32*' is not implicitly convertible to 'class A*'
+  // CHECK:STDERR: COMPILATION ERROR: fail_invalid_ptr_conversion2.carbon:[[@LINE+1]]: type error in initializer of variable: 'i32*' is not implicitly convertible to 'class A*'
   var c: A* = b;
   return 1;
 }

explorer/testdata/pointer/fail_rvalue_addressof.carbon

@@ -8,7 +8,7 @@ package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32 = 5;
-  // CHECK:STDERR: COMPILATION ERROR: explorer/testdata/pointer/fail_rvalue_addressof.carbon:[[@LINE+1]]: Argument to & should be a reference expression.
+  // CHECK:STDERR: COMPILATION ERROR: fail_rvalue_addressof.carbon:[[@LINE+1]]: Argument to & should be a reference expression.
   var p: i32* = &5;
   return 0;
 }

explorer/testdata/pointer/fail_use_after_free.carbon

@@ -9,6 +9,6 @@ package ExplorerTest api;
 fn Main() -> i32 {
   var p: i32* = heap.New(5);
   heap.Delete(p);
-  // CHECK:STDERR: RUNTIME ERROR: explorer/testdata/pointer/fail_use_after_free.carbon:[[@LINE+1]]: undefined behavior: access to dead value 5
+  // CHECK:STDERR: RUNTIME ERROR: fail_use_after_free.carbon:[[@LINE+1]]: undefined behavior: access to dead value 5
   return *p;
 }