fix: use prophecy for php generated unit tests

[jdpedrie]

PHPUnit 5 is still required by Google Cloud PHP since it has support for PHP 5. However in PHP 7.4 the PHPUnit mock builder raises the error Function ReflectionType::__toString() is deprecated, causing generated unit tests to fail.

This change switches to the Prophecy mock library (bundled with PHPUnit), which has better support for PHP 7.4.

[codecov[bot]]

Codecov Report

Merging #3225 into master will increase coverage by 0.00%. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3225   +/-   ##
=========================================
  Coverage     87.12%   87.12%           
+ Complexity     6080     6077    -3     
=========================================
  Files           494      494           
  Lines         24060    24052    -8     
  Branches       2613     2613           
=========================================
- Hits          20962    20956    -6     
+ Misses         2236     2234    -2     
  Partials        862      862           

Impacted Files	Coverage Δ	Complexity Δ
...om/google/api/codegen/gapic/ProtocGapicWriter.java	66.66% <0.00%> (-2.57%)	5.00% <0.00%> (ø%)
...rc/main/java/com/google/api/codegen/util/Name.java	82.35% <0.00%> (-2.17%)	57.00% <0.00%> (-3.00%)
...oogle/api/codegen/config/DiscoveryMethodModel.java	85.04% <0.00%> (-0.14%)	47.00% <0.00%> (-1.00%)
...main/java/com/google/api/codegen/util/Scanner.java	98.78% <0.00%> (-0.02%)	37.00% <0.00%> (ø%)
...a/com/google/api/codegen/util/MultiYamlReader.java	64.70% <0.00%> (ø)	5.00% <0.00%> (ø%)
...e/api/codegen/transformer/InitCodeTransformer.java	95.15% <0.00%> (+0.18%)	120.00% <0.00%> (+1.00%)
...om/google/api/codegen/config/ProtoMethodModel.java	89.55% <0.00%> (+1.31%)	35.00% <0.00%> (ø%)
...a/com/google/api/codegen/config/ProtoApiModel.java	94.73% <0.00%> (+2.42%)	18.00% <0.00%> (ø%)
...a/com/google/api/codegen/config/DiscoApiModel.java	79.31% <0.00%> (+2.64%)	9.00% <0.00%> (ø%)
...google/api/codegen/config/DiscoInterfaceModel.java	60.00% <0.00%> (+2.85%)	6.00% <0.00%> (ø%)
... and 1 more

---

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 dc019bd...9481acd. Read the comment docs.