fix(testing_util): link opentelemetry-cpp SDK as PUBLIC

The `opentelemetry_matchers.h` public header of `google_cloud_cpp_testing`
includes and uses opentelemetry-cpp SDK symbols inline (e.g.
`sdk::trace::SpanData::GetAttributes()`). Tests that link
`google_cloud_cpp_testing` therefore reference these SDK symbols directly
from their own translation units.

However, the CMake build links `opentelemetry-cpp::in_memory_span_exporter`
and `opentelemetry-cpp::trace` as PRIVATE, so they are not propagated to
the test executables. When building with shared libraries this fails to
link, e.g.:

    undefined reference to symbol
      '...sdk::trace::SpanData::GetAttributes[abi:cxx11]() const'
    libopentelemetry_trace.so: error adding symbols:
      DSO missing from command line

The Bazel build already exposes these dependencies transitively (via
`google_cloud_cpp_testing_private`), so making the CMake link PUBLIC also
aligns the two build systems.

Author: GaetanLepage

google/cloud/testing_util/CMakeLists.txt

@@ -58,8 +58,8 @@ add_library(
 target_link_libraries(
     google_cloud_cpp_testing
     PUBLIC absl::symbolize absl::failure_signal_handler google-cloud-cpp::common
-           GTest::gmock
-    PRIVATE opentelemetry-cpp::in_memory_span_exporter opentelemetry-cpp::trace)
+           GTest::gmock opentelemetry-cpp::in_memory_span_exporter
+           opentelemetry-cpp::trace)
 
 google_cloud_cpp_add_common_options(google_cloud_cpp_testing)