[Flang] Assign unittests to flang_compile pool

[Meinersbur]

#127364 added the FLANG_PARALLEL_COMPILE_JOBS option to limit the number of parallel compilations of Flang sources which require an unusal amount of memory. This patch also adds Flang's unittests to this pool which use about the same amount of memory.

[llvm-ci]

LLVM Buildbot has detected a new failure on builder premerge-monolithic-linux running on premerge-linux-1 while building flang at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/153/builds/39207

Here is the relevant piece of the build log for the reference

Step 7 (test-build-unified-tree-check-all) failure: test (failure)
...
PASS: lld :: COFF/arm64ec.test (98864 of 101865)
PASS: lld :: COFF/dtlto/files.test (98865 of 101865)
PASS: lld :: COFF/driver.test (98866 of 101865)
PASS: lld :: COFF/dtlto/options.test (98867 of 101865)
PASS: lld :: COFF/duplicate.test (98868 of 101865)
PASS: lld :: COFF/empty-subsection.s (98869 of 101865)
PASS: lld :: COFF/comdat-selection-associative-largest.s (98870 of 101865)
PASS: lld :: COFF/edata.s (98871 of 101865)
PASS: lld :: COFF/error-limit.test (98872 of 101865)
TIMEOUT: MLIR :: Examples/standalone/test.toy (98873 of 101865)
******************** TEST 'MLIR :: Examples/standalone/test.toy' FAILED ********************
Exit Code: 1
Timeout: Reached timeout of 60 seconds

Command Output (stdout):
--
# RUN: at line 1
"/etc/cmake/bin/cmake" "/build/buildbot/premerge-monolithic-linux/llvm-project/mlir/examples/standalone" -G "Ninja"  -DCMAKE_CXX_COMPILER=/usr/bin/clang++ -DCMAKE_C_COMPILER=/usr/bin/clang  -DLLVM_ENABLE_LIBCXX=OFF -DMLIR_DIR=/build/buildbot/premerge-monolithic-linux/build/lib/cmake/mlir  -DLLVM_USE_LINKER=lld  -DPython3_EXECUTABLE="/usr/bin/python3.10"
# executed command: /etc/cmake/bin/cmake /build/buildbot/premerge-monolithic-linux/llvm-project/mlir/examples/standalone -G Ninja -DCMAKE_CXX_COMPILER=/usr/bin/clang++ -DCMAKE_C_COMPILER=/usr/bin/clang -DLLVM_ENABLE_LIBCXX=OFF -DMLIR_DIR=/build/buildbot/premerge-monolithic-linux/build/lib/cmake/mlir -DLLVM_USE_LINKER=lld -DPython3_EXECUTABLE=/usr/bin/python3.10
# .---command stdout------------
# | -- The CXX compiler identification is Clang 16.0.6
# | -- The C compiler identification is Clang 16.0.6
# | -- Detecting CXX compiler ABI info
# | -- Detecting CXX compiler ABI info - done
# | -- Check for working CXX compiler: /usr/bin/clang++ - skipped
# | -- Detecting CXX compile features
# | -- Detecting CXX compile features - done
# | -- Detecting C compiler ABI info
# | -- Detecting C compiler ABI info - done
# | -- Check for working C compiler: /usr/bin/clang - skipped
# | -- Detecting C compile features
# | -- Detecting C compile features - done
# | -- Looking for histedit.h
# | -- Looking for histedit.h - found
# | -- Found LibEdit: /usr/include (found version "2.11") 
# | -- Found ZLIB: /usr/lib/x86_64-linux-gnu/libz.so (found version "1.2.11") 
# | -- Found LibXml2: /usr/lib/x86_64-linux-gnu/libxml2.so (found version "2.9.13") 
# | -- Using MLIRConfig.cmake in: /build/buildbot/premerge-monolithic-linux/build/lib/cmake/mlir
# | -- Using LLVMConfig.cmake in: /build/buildbot/premerge-monolithic-linux/build/lib/cmake/llvm
# | -- Linker detection: unknown
# | -- Performing Test LLVM_LIBSTDCXX_MIN
# | -- Performing Test LLVM_LIBSTDCXX_MIN - Success
# | -- Performing Test LLVM_LIBSTDCXX_SOFT_ERROR
# | -- Performing Test LLVM_LIBSTDCXX_SOFT_ERROR - Success
# | -- Performing Test CXX_SUPPORTS_CUSTOM_LINKER
# | -- Performing Test CXX_SUPPORTS_CUSTOM_LINKER - Success
# | -- Performing Test C_SUPPORTS_FPIC
# | -- Performing Test C_SUPPORTS_FPIC - Success
# | -- Performing Test CXX_SUPPORTS_FPIC