[lldb][test] Remove symlink for API tests.

Summary: Moves lldbsuite tests to lldb/test/API.

This is a largely mechanical change, moved with the following steps:

```
rm lldb/test/API/testcases
mkdir -p lldb/test/API/{test_runner/test,tools/lldb-{server,vscode}}
mv lldb/packages/Python/lldbsuite/test/test_runner/test lldb/test/API/test_runner
for d in $(find lldb/packages/Python/lldbsuite/test/* -maxdepth 0 -type d | egrep -v "make|plugins|test_runner|tools"); do mv $d lldb/test/API; done
for d in $(find lldb/packages/Python/lldbsuite/test/tools/lldb-vscode -maxdepth 1 -mindepth 1 | grep -v ".py"); do mv $d lldb/test/API/tools/lldb-vscode; done
for d in $(find lldb/packages/Python/lldbsuite/test/tools/lldb-server -maxdepth 1 -mindepth 1 | egrep -v "gdbremote_testcase.py|lldbgdbserverutils.py|socket_packet_pump.py"); do mv $d lldb/test/API/tools/lldb-server; done
```

lldb/packages/Python/lldbsuite/__init__.py and lldb/test/API/lit.cfg.py were also updated with the new directory structure.

Reviewers: labath, JDevlieghere

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71151

lldb/packages/Python/lldbsuite/__init__.py

@@ -22,11 +22,4 @@ lldb_root = find_lldb_root()
 
 # lldbsuite.lldb_test_src_root refers to the root of the python test case tree
 # (i.e. the actual unit tests).
-lldb_test_root = os.path.join(
-    lldb_root,
-    "packages",
-    "Python",
-    "lldbsuite",
-    "test")
-# TODO(rupprecht): update the above definition after moving test cases:
-# lldb_test_root = os.path.join(lldb_root, "test", "API")
+lldb_test_root = os.path.join(lldb_root, "test", "API")