[TEST] Fix sync problems in OTLP File exporter tests. #10087

Job	Run time
CMake test arm64 (with modern protobuf,grpc and abseil)	17m 47s
CMake clang 18 (maintainer mode, abiv2)	8m 26s
CMake clang 18 (maintainer mode, async)	8m 22s
CMake clang 18 (maintainer mode, sync)	8m 25s
CMake gcc 14 (maintainer mode, async)	12m 45s
CMake gcc 14 (maintainer mode, sync)	13m 3s
CMake msvc (maintainer mode)	6m 37s
CMake test (without otlp-exporter)	5m 34s
CMake test (with abseil)	5m 56s
CMake do not install test (with otlp-exporter)	16m 32s
CMake test (with modern protobuf,grpc and abseil)	18m 37s
CMake msvc (maintainer mode) with C++20	8m 38s
CMake test (with opentracing-shim)	4m 37s
CMake test (with otlp-exporter)	14m 46s
CMake C++14 test(GCC)	4m 28s
CMake C++17 test(GCC)	6m 41s
CMake C++20 test(Clang with libc++)	9m 43s
CMake C++20 test(GCC)	9m 11s
CMake C++23 test(Clang with libc++)	9m 28s
CMake C++23 test(GCC)	9m 8s
CMake test (without otlp-exporter and with async export)	6m 1s
Bazel asan config	21m 30s
Bazel without bzlmod	16m 54s
Bazel noexcept	16m 3s
Bazel nortti	16m 58s
Bazel on MacOS	13m 1s
Bazel	17m 7s
Bazel with async export	17m 1s
Bazel tsan config	17m 36s
Bazel valgrind	7m 3s
Benchmark	13m 50s
CMake install test (with abseil)	6m 8s
CMake test (build shared libraries with otlp-exporter and static gRPC)	15m 45s
Copyright	16s
Format	1m 0s
Plugin -> CMake	59s
Code coverage	11m 43s
DocFX check	56s
markdown-lint	9s
misspell	5s
shellcheck	22s
CMake -> exporter proto	13m 15s
Bazel Windows	34m 8s
Plugin -> CMake Windows	7m 36s
CMake (With async export) -> exporter proto	12m 45s
CMake -> exporter proto (Build as DLL)	28m 4s
	8h 14m 59s

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