meson: Enable testing in CI (#1609)

meson: Enable testing in CI (#1609)

* tests: Fix shellcheck warnings in playTests.sh

* tests: Do not use ../programs which is relative to tests dirs

This commit fixes error when running playTests.sh in Meson.
Mesonbuild runs out of tree, so ./datagen not in `zstd/tests` dir,
it lies in <mesonbuilddir>/tests. This leads to ../programs invalid.

* tests: Replace relative paths for zstd/tests dir

* playTests: Set shell options explicitly, not in shebang

* playTests: Replace echo -e with printf

* meson: Fix test-zstd

Use std=gnu99 to build and test just like `make test`.

* meson: Fix legacy test

* meson: Enable testing in CI

Run build under release mode for faster test time.

* meson: Increase timeout time for test-zstream