chore: system tests for gax

[alexander-fenster]

Introducing system tests for gax. The test does the following:

• clones video intelligence, Pub/Sub, and Speech, and checks out their latest releases;
• npm link local gax;
• run system tests for the cloned packages;
• display coverage.

I expect it will make updating and/or refactoring gax less scary.

Changes to CircleCI config for synthtool: googleapis/synthtool#114.

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[alexander-fenster]

It even works in Kokoro!

FYI, here is the coverage of gax code by those system tests:

-------------------------|----------|----------|----------|----------|-------------------|
File                     |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
-------------------------|----------|----------|----------|----------|-------------------|
All files                |    39.15 |    21.91 |    43.98 |    36.98 |                   |
 GoogleError.ts          |      100 |      100 |      100 |      100 |                   |
 api_callable.ts         |    70.09 |    53.06 |    76.67 |    68.18 |... 52,359,403,411 |
 bundling.ts             |    13.46 |        0 |    12.12 |     11.5 |... 70,571,572,583 |
 gax.ts                  |     68.8 |    49.51 |    64.29 |    68.29 |... 63,665,666,669 |
 grpc.ts                 |    71.77 |    46.51 |       75 |    72.17 |... 58,360,361,363 |
 index.ts                |      100 |      100 |      100 |      100 |                   |
 longrunning.ts          |    78.47 |    61.02 |    86.21 |    77.37 |... 64,369,370,429 |
 operations_client.ts    |    65.79 |    16.67 |    64.29 |    63.89 |... 30,432,433,436 |
 paged_iteration.ts      |    24.47 |        5 |    33.33 |    21.98 |... 07,208,209,212 |
 parser_extras.ts        |    23.08 |        0 |        0 |    23.08 |... 94,113,114,115 |
 path_template.ts        |    16.22 |        0 |     9.09 |    15.07 |... 89,190,192,195 |
 path_template_parser.ts |      1.8 |        0 |     7.69 |      1.8 |... 30,632,633,636 |
 routing_header.ts       |    33.33 |      100 |        0 |       50 |                50 |
 streaming.ts            |    37.18 |     8.33 |    31.82 |    34.72 |... 96,200,204,208 |
-------------------------|----------|----------|----------|----------|-------------------|

[JustinBeckwith]

This is freaking awesome.