Use a common object library to avoid repeated compilation when building C++ tests

[ttnghia]

Currently, building each test executable involves compiling all the source files from cudf::cudftestutil_impl. That is because cudf::cudftestutil_impl is just an interface library that does not compile but just provides a list of source files. This PR adds an object library that compiles these source files, allowing them to be compiled just once instead of compiled for every build of a test executable.

Closes #2506.

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Example of compiling column_utilities.cu from cudf::cudftestutil_impl:

Before:

cat build.ninja | grep column_utilities.cu.o: | wc -l
17

That means the file is compiled 17 times.

After:

cat build.ninja | grep column_utilities.cu.o: | wc -l
1

[ttnghia]

build