Test initialization refactor

[schmidt-sebastian]

This PR is virtually unreviewable, but is meant to make the next PR reviewable.

The follow up PR will change the way that client initialization works. Currently, the Server SDK has a single member variable (_firestoreClient) that stores the Veneer client. All tests use _firestoreClient to to add request assertions.

In order to support more than 100 concurrent listeners, a follow up PR will change this and support an arbitrary number of clients. The tests will then no longer be able to rely on _firestoreClient and won't be able to override request handlers directly. Instead, they will override the logic that creates these clients: https://github.com/googleapis/nodejs-firestore/pull/242/files#diff-51dde1525cb3ea0c2eea78ebf10ea7e2R84

While the second PR will add actual changes to the client, this PR just changes all of the tests to use a new helper method to init the client (look in test/util/helpers.ts). This required initialization and formatting changes all over the test code. These changes all follow the same pattern, and probably don't need to be reviewed in detail each time.

There is one logic change in the client to make the test work: With the new initialization, some of the conformance tests no longer pass since they can't tell that two paths are equal if one already has _formattedName initialized. I got rid of the lazy-loading here.

[codecov]

Codecov Report

Merging #254 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #254   +/-   ##
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  Coverage     100%   100%           
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  Files          12     12           
  Lines        2104   2104           
  Branches      457    457           
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  Hits         2104   2104

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[var-const]

LGTM with some significant caveats:

• I don't have enough context to really judge the high-level logic. I mainly checked that the code does implement that logic.
• the PR is huge and I might have easily missed something among the similar changes.

[schmidt-sebastian]

Changed base branch to master and added @stephenplusplus for owners approval.