fix: Set TEXTPLAN_SOURCE_DIR based on current dir in planloader test. (…

…#101)

That variable was previously computed based on `CMAKE_SOURCE_DIR`, which
is the wrong value if `substrait-cpp` is used as a dependency from a
different top-level CMake project. The PR applies the pattern based on
`CMAKE_CURRENT_SOURCE_DIR` used in other tests.

export/planloader/tests/CMakeLists.txt

@@ -15,7 +15,8 @@ add_test_case(
   gtest
   gtest_main)
 
-set(TEXTPLAN_SOURCE_DIR "${CMAKE_SOURCE_DIR}/src/substrait/textplan")
+set(TEXTPLAN_SOURCE_DIR
+    "${CMAKE_CURRENT_SOURCE_DIR}/../../../src/substrait/textplan")
 
 add_custom_command(
   TARGET planloader_test