test: validate explicit routing

[dbolduc]

Fixes #9373

Note that any test in Bigtable (the only service with explicit routing for which we have a library) which sets an app profile id will now fail the context metadata check. There is apparently exactly one such test, so we implement a temporary workaround.

My plan is to add more such tests by also setting the app profile id in the bigtable_stub_factory_test.cc.
But this will have to wait until after the generator has explicit routing support.

google-cloud-cpp/google/cloud/bigtable/internal/bigtable_stub_factory_test.cc

Lines 117 to 119 in 91aae8c

	google::bigtable::v2::ReadRowsRequest req;
	req.set_table_name(
	"projects/the-project/instances/the-instance/tables/the-table");

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[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #9385 (d8fff54) into main (55f0da0) will decrease coverage by 0.00%.
The diff coverage is 84.84%.

@@            Coverage Diff             @@
##             main    #9385      +/-   ##
==========================================
- Coverage   94.68%   94.68%   -0.01%     
==========================================
  Files        1480     1480              
  Lines      135457   135483      +26     
==========================================
+ Hits       128258   128276      +18     
- Misses       7199     7207       +8     

Impacted Files	Coverage Δ
google/cloud/testing_util/validate_metadata.cc	91.48% <81.48%> (-2.38%)	⬇️
.../cloud/bigtable/internal/legacy_row_reader_test.cc	100.00% <100.00%> (ø)
...loud/bigtable/internal/connection_refresh_state.cc	97.22% <0.00%> (-2.78%)	⬇️
...integration_tests/schema_admin_integration_test.cc	98.88% <0.00%> (-1.12%)	⬇️

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