Support running a specific test/suite in boa_tester

[georgeroman]

These changes add support for running a specific test or suite in boa_tester. For example, specifying -s test/built-ins/BigInt will only run the BigInt suite, while specifying -s test/built-ins/BigInt/asIntN/arithmetic.js will only run the specified test. If nothing gets specified, it defaults to running the full suite.

[codecov]

Codecov Report

Merging #886 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #886      +/-   ##
==========================================
- Coverage   59.30%   59.29%   -0.02%     
==========================================
  Files         165      165              
  Lines       10376    10378       +2     
==========================================
  Hits         6154     6154              
- Misses       4222     4224       +2     

Impacted Files	Coverage Δ
boa_tester/src/main.rs	0.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 71bc080...af16395. Read the comment docs.

[Razican]

This is very nice indeed. Thank you!