ARROW-7450: [C++] Also link boost_filesystem when using static test linkage

The nightly test-ubuntu-18.04-cpp-static has been failing due to a missing Boost link dependency in the unit tests

Closes #6130 from wesm/ARROW-7450 and squashes the following commits:

8984f41 <Wes McKinney> Remove whitespace
bd802c3 <Wes McKinney> Also link boost_filesystem when using static test linkage

Authored-by: Wes McKinney <[email protected]>
Signed-off-by: Neal Richardson <[email protected]>

Author: wesm

cpp/CMakeLists.txt

@@ -696,7 +696,12 @@ if(NOT MSVC)
   list(APPEND ARROW_SHARED_INSTALL_INTERFACE_LIBS ${CMAKE_DL_LIBS})
 endif()
 
-set(ARROW_TEST_LINK_TOOLCHAIN GTest::Main GTest::GTest GTest::GMock)
+set(ARROW_TEST_LINK_TOOLCHAIN
+    GTest::Main
+    GTest::GTest
+    GTest::GMock
+    ${BOOST_FILESYSTEM_LIBRARY}
+    ${BOOST_SYSTEM_LIBRARY})
 
 if(ARROW_BUILD_TESTS)
   add_dependencies(arrow_test_dependencies ${ARROW_TEST_LINK_TOOLCHAIN})
@@ -711,13 +716,8 @@ endif()
 set(ARROW_TEST_STATIC_LINK_LIBS arrow_testing_static arrow_static ${ARROW_LINK_LIBS}
                                 ${ARROW_TEST_LINK_TOOLCHAIN})
 
-set(ARROW_TEST_SHARED_LINK_LIBS
-    arrow_testing_shared
-    arrow_shared
-    ${ARROW_LINK_LIBS}
-    ${BOOST_FILESYSTEM_LIBRARY}
-    ${BOOST_SYSTEM_LIBRARY}
-    ${ARROW_TEST_LINK_TOOLCHAIN})
+set(ARROW_TEST_SHARED_LINK_LIBS arrow_testing_shared arrow_shared ${ARROW_LINK_LIBS}
+                                ${ARROW_TEST_LINK_TOOLCHAIN})
 
 if(NOT MSVC)
   set(ARROW_TEST_SHARED_LINK_LIBS ${ARROW_TEST_SHARED_LINK_LIBS} ${CMAKE_DL_LIBS})