Add fuzzer test for scalar numbers in json. (#4996)

* Add fuzzer test for scalar numbers in json. Grammar-based regex used to check correctness.

* Fix conversation

tests/fuzzer/readme.md

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+# Test Flatbuffers library with help of libFuzzer
+Test suite of Flatbuffers library has fuzzer section with tests are based on libFuzzer library.
+
+> LibFuzzer is in-process, coverage-guided, evolutionary fuzzing engine.
+LibFuzzer is linked with the library under test, and feeds fuzzed inputs to the library via a specific fuzzing entrypoint (aka “target function”);
+the fuzzer then tracks which areas of the code are reached, and generates mutations on the corpus of input data in order to maximize the code coverage.
+The code coverage information for libFuzzer is provided by LLVM’s SanitizerCoverage instrumentation.
+
+For details about **libFuzzer** see: https://llvm.org/docs/LibFuzzer.html
+
+To build and run these tests LLVM compiler (with clang frontend) and CMake should be installed before.
+
+The fuzzer section include three tests:
+- `verifier_fuzzer` checks stability of deserialization engine for `Monster` schema;
+- `parser_fuzzer` checks stability of schema and json parser under various inputs;
+- `scalar_parser` focused on validation of the parser while parse numeric scalars in schema and/or json files;
+
+## Build tests with locales
+Flatbuffers library use only printable-ASCII characters as characters of grammar alphabet for type and data declaration.
+This alphabet is fully compatible with JSON specification and make schema declaration fully portable.
+Flatbuffers library is independent from global or thread locales used by end-user application.
+To run fuzzer tests with selected C-locale under test pass `-DFUZZ_TEST_LOCALE="<locale name>"` to CMake when configuring.
+Selected locale must be installed in system before use.
+Command line:
+```sh
+cmake .. -DFUZZ_TEST_LOCALE="ru_RU.CP1251"
+```
+If use VSCode, use `cmake.configureSettings` section of workspace settings:
+```json
+"cmake.configureSettings": {
+  "FUZZ_TEST_LOCALE" : "ru_RU.CP1251"
+}
+```
+
+## Run fuzzer
+These are examples of fuzzer run.
+Flags may vary and depend from version of libFuzzer library.
+For detail, run a fuzzer test with help flag: `./parser_fuzzer -help=1`
+
+`./verifier_fuzzer -reduce_depth=1 -use_value_profile=1 -shrink=1 ../.corpus_verifier/`
+
+`./parser_fuzzer -reduce_depth=1 -use_value_profile=1 -shrink=1 ../.corpus_parser/`
+
+`./scalar_fuzzer -reduce_depth=1 -use_value_profile=1 -shrink=1 -max_len=3000 ../.corpus_parser/ ../.seed_parser/`
+
+Flag `-only_ascii=1` is useful for fast number-compatibility checking while run `scalar_fuzzer`:
+
+`./scalar_fuzzer -only_ascii=1 -reduce_depth=1 -use_value_profile=1 -shrink=1 -max_len=3000 -timeout=10 -rss_limit_mb=2048 -jobs=2 ../.corpus_parser/ ../.seed_parser/`
+
+## Merge (minimize) corpus
+The **libFuzzer** allow to filter (minimize) corpus with help of `-merge` flag:
+> -merge
+    If set to 1, any corpus inputs from the 2nd, 3rd etc. corpus directories that trigger new code coverage will be merged into the first corpus directory.
+    Defaults to 0. This flag can be used to minimize a corpus.
+
+Merge several seeds to one:
+`./scalar_fuzzer -merge=1 ../.corpus/ ../.seed_1/ ../.seed_2/`
+
+## Know limitations
+- LLVM 7.0 std::regex library has problem with stack overflow, maximum length of input for `scalar_fuzzer` run should be limited to 3000.
+  Example: `./scalar_fuzzer -max_len=3000`