Tokenizer test (#21)

Summary: Pull Request resolved: pytorch-labs/tokenizers#21

Test Plan:
## OSS 
Build
```
cmake .     -DCMAKE_INSTALL_PREFIX=cmake-out -DTOKENIZERS_BUILD_TEST=ON -Bcmake-out
cmake --build cmake-out -j9 --target install
```

Test
```
(executorch) [[email protected] /data/users/lfq/tokenizers/cmake-out (lfq.tokenizer-test)]$ ctest
Test project /data/users/lfq/tokenizers/cmake-out
    Start 1: test_base64
1/5 Test pytorch-labs/tokenizers#1: test_base64 ......................   Passed    0.00 sec
    Start 2: test_llama2c_tokenizer
2/5 Test pytorch-labs/tokenizers#2: test_llama2c_tokenizer ...........   Passed    0.00 sec
    Start 3: test_pre_tokenizer
3/5 Test pytorch-labs/tokenizers#3: test_pre_tokenizer ...............   Passed    0.73 sec
    Start 4: test_sentencepiece
4/5 Test pytorch-labs/tokenizers#4: test_sentencepiece ...............   Passed    0.04 sec
    Start 5: test_tiktoken
5/5 Test pytorch-labs/tokenizers#5: test_tiktoken ....................   Passed    3.32 sec

100% tests passed, 0 tests failed out of 5

Total Test time (real) =   4.10 sec
```

## Internal
```
 buck2 test fbsource//xplat/pytorch/tokenizers/test:
 buck2 test fbcode//pytorch/tokenizers/test:
```

Differential Revision: D69860352

Pulled By: lucylq

Author: lucylq

shim/xplat/executorch/build/runtime_wrapper.bzl

@@ -171,7 +171,7 @@ def _patch_kwargs_common(kwargs):
     # don't pick up unexpected clients while things are still in flux.
     if not kwargs.pop("_is_external_target", False):
         for target in kwargs.get("visibility", []):
-            if not (target.startswith("//executorch") or target.startswith("@")):
+            if not (target.startswith("//executorch") or target.startswith("//pytorch/tokenizers") or target.startswith("@")):
                 fail("Please manage all external visibility using the " +
                      "EXECUTORCH_CLIENTS list in " +
                      "//executorch/build/fb/clients.bzl. " +

test/utils/targets.bzl

@@ -21,6 +21,7 @@ def define_common_targets():
             ],
             visibility = [
                 "//executorch/...",
+                "//pytorch/tokenizers/...",
                 "@EXECUTORCH_CLIENTS",
             ],
             deps = [