Fix CMake build in test subdirectories

Due to the regexes, ortools_pdlp fails to build if any of its parent
directories starts with `test` or `gtest`:

    CMake Error at test/ortools-src/ortools/pdlp/CMakeLists.txt:27 (add_library):
      No SOURCES given to target: ortools_pdlp

ortools/pdlp/CMakeLists.txt

@@ -16,9 +16,9 @@ if(NOT USE_PDLP)
 endif()
 
 file(GLOB _SRCS "*.h" "*.cc")
-list(FILTER _SRCS EXCLUDE REGEX ".*/.*_test.cc")
-list(FILTER _SRCS EXCLUDE REGEX ".*/gtest.*")
-list(FILTER _SRCS EXCLUDE REGEX ".*/test.*")
+list(FILTER _SRCS EXCLUDE REGEX "/[^/]*_test\\.cc$")
+list(FILTER _SRCS EXCLUDE REGEX "/gtest[^/]*$")
+list(FILTER _SRCS EXCLUDE REGEX "/test[^/]*$")
 
 set(NAME ${PROJECT_NAME}_pdlp)