CI: Add automated test coverage reports.

[TriplEight]

Code coverage with grcov -> codecov.io

[codecov-io]

Codecov Report

Merging #276 into master will increase coverage by 54.43%.
The diff coverage is 25.58%.

@@             Coverage Diff             @@
##           master     #276       +/-   ##
===========================================
+ Coverage   18.74%   73.18%   +54.43%     
===========================================
  Files          28       99       +71     
  Lines        1814     7405     +5591     
===========================================
+ Hits          340     5419     +5079     
- Misses       1474     1986      +512     

Impacted Files	Coverage Δ
core/src/env/engine/off_chain/db/accounts.rs	80.39% <0.00%> (ø)
core/src/env/engine/off_chain/db/chain_spec.rs	72.00% <0.00%> (ø)
core/src/env/engine/off_chain/db/console.rs	0.00% <0.00%> (ø)
core/src/env/engine/off_chain/db/events.rs	0.00% <0.00%> (ø)
core/src/env/engine/off_chain/runtime_calls.rs	50.00% <0.00%> (ø)
core/src/env/engine/off_chain/runtime_storage.rs	57.14% <0.00%> (ø)
lang/macro/src/codegen/testable.rs	84.00% <ø> (+84.00%)	⬆️
core/src/env/engine/off_chain/mod.rs	67.08% <42.10%> (ø)
core/src/env/engine/off_chain/test_api.rs	62.79% <100.00%> (ø)
primitives/src/key.rs	59.67% <0.00%> (-10.96%)	⬇️
... and 94 more

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[TriplEight]

Apparently, -Zno-landing-pads caused test failures, removed it.
This is how codecov is working now.
https://gitlab.parity.io/parity/ink/-/jobs/483361
https://codecov.io/gh/paritytech/ink/tree/dccf7f18e53ad7f0e517b42227173adde723ba04
@Robbepop I need to know if it counts an adequate amount of lines and approximate coverage.

[TriplEight]

This is what I've managed to reach with grcov https://codecov.io/gh/paritytech/ink/tree/84e452a2b08d8ca6a7ed5c26a17d61ed7e183615

This is how to run tarpaulin, which is better, but there's no way to run it within docker not dropping it's security via
docker run -it --security-opt seccomp=unconfined ...

cargo install cargo-tarpaulin
CODECOV_TOKEN=xxxxxxxxxxxxxxxxxx cargo +nightly tarpaulin --verbose --all-features --workspace -t 120 --out Xml
CODECOV_TOKEN=xxxxxxxxxxxxxxxxxx bash <(curl -s https://codecov.io/bash)