cmake: make `random_mocking_bit_gen` library public. (#1084)

An external project may depend on an Abseil `TESTONLY` library like
`absl::random_mocking_bit_gen`.  Such a library should not be excluded
from build when Abseil's own tests are not built.

fixes #997

CMake/AbseilHelpers.cmake

@@ -83,7 +83,8 @@ function(absl_cc_library)
     ${ARGN}
   )
 
-  if(ABSL_CC_LIB_TESTONLY AND NOT (BUILD_TESTING AND ABSL_BUILD_TESTING))
+  if(NOT ABSL_CC_LIB_PUBLIC AND ABSL_CC_LIB_TESTONLY AND
+      NOT (BUILD_TESTING AND ABSL_BUILD_TESTING))
     return()
   endif()
 

absl/random/CMakeLists.txt

@@ -121,6 +121,7 @@ absl_cc_library(
     absl::variant
     GTest::gmock
     GTest::gtest
+  PUBLIC
   TESTONLY
 )