Account for placeholders in additional bindings

[dwsupplee]

For more background: googleapis/google-cloud-php#1411

/cc @tmatsuo @jdpedrie

[codecov-io]

Codecov Report

Merging #2416 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #2416      +/-   ##
============================================
+ Coverage     86.72%   86.72%   +<.01%     
  Complexity     5241     5241              
============================================
  Files           458      458              
  Lines         20870    20873       +3     
  Branches       2252     2252              
============================================
+ Hits          18099    18102       +3     
  Misses         1968     1968              
  Partials        803      803

Impacted Files	Coverage Δ	Complexity Δ
...en/transformer/php/PhpGapicSurfaceTransformer.java	93.03% <100%> (+0.07%)	41 <0> (ø)	⬇️

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[michaelbausor]

Can we also update our tests so that this change is visible in the baseline file? I presume we will need to modify an existing method in our test proto (e.g. https://github.com/googleapis/gapic-generator/blob/master/src/test/java/com/google/api/codegen/testsrc/libraryproto/library.proto#L130) so that it has an additional binding with a different placeholder structure.

[dwsupplee]

@michaelbausor Good thought. I've added 0a9d59d and 92b1644. PTAL and let me know what you think when you have a moment.