fix: Set up DirectPath e2e tests correctly

[mohanli-ml]

In #770 we make the client to attempt DirectPath by default using the default spanner endpoint spanner.googleapis.com. However, currently DirectPath tests still need to use a custom endpoint aa423245250f2bbf.sandbox.googleapis.com, so we need to set it to a customChannelProvider, which will override the defaultChannelProvider in https://github.com/googleapis/java-spanner/blob/master/google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpc.java#L336. As a result, DirectPath tests can not be run properly. This PR fixes this problem by setting up the customChannelProvider properly.

[codecov]

Codecov Report

Merging #780 (b750278) into master (beeac09) will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #780      +/-   ##
============================================
+ Coverage     85.01%   85.05%   +0.04%     
- Complexity     2561     2564       +3     
============================================
  Files           143      143              
  Lines         14005    14005              
  Branches       1337     1337              
============================================
+ Hits          11906    11912       +6     
+ Misses         1537     1532       -5     
+ Partials        562      561       -1     

Impacted Files	Coverage Δ	Complexity Δ
.../google/cloud/spanner/SpannerExceptionFactory.java	83.50% <0.00%> (+1.03%)	47.00% <0.00%> (+1.00%)
...m/google/cloud/spanner/connection/FailedQuery.java	82.75% <0.00%> (+6.89%)	3.00% <0.00%> (+1.00%)
...r/AbortedDueToConcurrentModificationException.java	85.71% <0.00%> (+42.85%)	2.00% <0.00%> (+1.00%)

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