Simplify test suite configuration

[morozov]

Q	A
Type	improvement
BC Break	no

1. None of the supported drivers require all the connection parameters required by the test suite configuration. Most of them have default values for most of the parameters (e.g. mysqli can connect even w/o any parameters). Having to specify them all in a local development environment is unnecessarily tedious.
2. All so-called temporary connections look identical to the primary ones with the exception that the temporary one doesn't usually have a database name specified (creating a database is the primary purpose of the temporary connection). Instead of requiring the parameters to be specified explicitly, in the absence of the parameters, the test suite can fall back to the primary connection parameters and ignore the database.
3. There's no exact match between the test suite configuration parameter names and the connection parameters. E.g. the the driver is configured as db_type, dbname is configured as db_name. It would be nice to have a direct correspondence.

BTW, tmp_host, tmp_port, etc. also look redundant.

Change summary

1. (tmp)db_type → (tmp)db_driver
2. (tmp)db_username → (tmp)db_user. This isn't nice but it corresponds to the connection parameter name.
3. (tmp)db_name → (tmp)db_dbname. This isn't nice either, but this is because we're not configuring a database (db_), we're configuring a test database connection. We may get rid of the db_ prefix in parameter names or replace it with something else (e.g. test_).
4. Connection parameters are reordered logically: driver, host, port, user, password.

[codecov-commenter]

Codecov Report

Merging #4067 into 2.10.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             2.10.x    #4067      +/-   ##
============================================
- Coverage     73.32%   73.30%   -0.03%     
  Complexity     5031     5031              
============================================
  Files           215      215              
  Lines         12825    12825              
============================================
- Hits           9404     9401       -3     
- Misses         3421     3424       +3     

Impacted Files	Coverage Δ	Complexity Δ
lib/Doctrine/DBAL/Driver/SQLSrv/Driver.php	84.21% <0.00%> (-5.27%)	9.00% <0.00%> (ø%)
lib/Doctrine/DBAL/Driver/PDOSqlsrv/Driver.php	92.59% <0.00%> (-3.71%)	13.00% <0.00%> (ø%)
lib/Doctrine/DBAL/Driver/PDOPgSql/Driver.php	84.61% <0.00%> (-2.57%)	20.00% <0.00%> (ø%)

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[morozov]

The code coverage slightly dropped because the pdo_pgsql and *sqlsrv drivers no longer use the port to build the DSN. It's better to test building DSN later in a unit test rather than have it in the connection parameters to have unintended coverage.