Add testing of C++ with sanitizers (CI-Docker) (#5631)

* Add C++ build testing with clang and gcc

This adds Dockerfiles which test building flatc and the C++ library against clang
and gcc. See discussion at #5119.  It is derived from the Travis CI tooling.

The GRPC tests are failing due to #5099 so those are commented out.

These are run from the .travis.yml file rather than the tests/docker/languages
folder because the builds may each take longer than 30 minutes and were hitting
Travis timeouts.

Parallel builds and build caching attempt to keep the build times low.

* Add GCC 8.3 and Clang 7.0 with sanitizers into CI (based on #5130)

- Add a docker based on Debian Buster.
- Add C++ building scripts for the docker.
- Leak-sanitizer requires SYS_PTRACE.

tests/docker/Dockerfile.testing.cpp.debian_buster

@@ -0,0 +1,10 @@
+FROM debian:10.1-slim as base
+RUN apt -qq update >/dev/null
+RUN apt -qq install -y cmake make build-essential >/dev/null
+RUN apt -qq install -y autoconf git libtool >/dev/null
+RUN apt -qq install -y clang >/dev/null
+FROM base
+# Travis machines have 2 cores. Can be redefined with 'run --env PAR_JOBS=N'.
+ENV JOBS=2
+WORKDIR /flatbuffers
+ADD . .