Switch test manifests to embedded C++ (#5036)

Inconsistent execution environments make using a path-as-define
difficult, so switch to an embedded file.

Also fixes the lldb launch so that passing tests run cleanly, and adds
TEST_TARGET to gdb (but without testing there). I'm dropping `sourceMap`
because it's not handled quite correctly (also not great to be trying to
pass source mappings in two different ways), and `env` didn't seem to be
working as intended either; maybe specifying `initCommands` causes other
things to not be evaluated. But the straight `initCommands` looks like
it's working. I used lldb to validate execution of these changes.

```
Running initCommands:
(lldb) command script import external/+llvm_project+llvm-project/llvm/utils/lldbDataFormatters.py
(lldb) settings set target.source-map "." "/usr/local/google/home/jperkins/dev/carbon-lang"
(lldb) settings set target.source-map "/proc/self/cwd" "/usr/local/google/home/jperkins/dev/carbon-lang"
(lldb) env TEST_TARGET=//toolchain/testing:file_test
(lldb) env TEST_TMPDIR=/tmp
Running tests with 128 thread(s)
.
Done!
Note: Google Test filter = ToolchainFileTest.toolchain/check/testdata/const/collapse.carbon
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from ToolchainFileTest
[ RUN      ] ToolchainFileTest.toolchain/check/testdata/const/collapse.carbon
[       OK ] ToolchainFileTest.toolchain/check/testdata/const/collapse.carbon (0 ms)
[----------] 1 test from ToolchainFileTest (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (1 ms total)
[  PASSED  ] 1 test.
Process 3869310 exited with status = 0 (0x00000000) 
```

testing/file_test/file_test_base.cpp

@@ -29,7 +29,6 @@
 #include "absl/flags/flag.h"
 #include "absl/flags/parse.h"
 #include "absl/strings/str_join.h"
-#include "absl/strings/str_split.h"
 #include "common/check.h"
 #include "common/error.h"
 #include "common/exe_path.h"
@@ -42,7 +41,6 @@
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/ThreadPool.h"
-#include "testing/base/file_helpers.h"
 #include "testing/file_test/autoupdate.h"
 #include "testing/file_test/run_test.h"
 #include "testing/file_test/test_file.h"
@@ -298,14 +296,9 @@ static auto RegisterTests(FileTestFactory* test_factory,
                           llvm::StringRef exe_path,
                           llvm::SmallVectorImpl<FileTestInfo>& tests)
     -> ErrorOr<Success> {
-  GetFileTestManifestPath();
-  CARBON_ASSIGN_OR_RETURN(auto test_manifest,
-                          ReadFile(GetFileTestManifestPath()));
-
   // Prepare the vector first, so that the location of entries won't change.
-  for (const auto& test_name :
-       absl::StrSplit(test_manifest, "\n", absl::SkipEmpty())) {
-    tests.push_back({.test_name = std::string(test_name)});
+  for (auto& test_name : GetFileTestManifest()) {
+    tests.push_back({.test_name = test_name});
   }
 
   // Amend entries with factory functions.