[libc] Add a new suite called "libc-long-running-tests".

This suite is helpful is adding long running tests which take a long
time to finish that they can be run on the public builders. They
will probably be run on special builders in future.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D104816

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -178,6 +178,11 @@ function(add_libc_exhaustive_testsuite suite_name)
   add_dependencies(exhaustive-check-libc ${suite_name})
 endfunction(add_libc_exhaustive_testsuite)
 
+function(add_libc_long_running_testsuite suite_name)
+  add_custom_target(${suite_name})
+  add_dependencies(libc-long-running-tests ${suite_name})
+endfunction(add_libc_long_running_testsuite)
+
 # Rule to add a fuzzer test.
 # Usage
 #    add_libc_fuzzer(

libc/test/CMakeLists.txt

@@ -9,6 +9,7 @@ add_custom_target(check-llvmlibc)
 add_dependencies(check-libc check-llvmlibc)
 
 add_custom_target(exhaustive-check-libc)
+add_custom_target(libc-long-running-tests)
 
 add_subdirectory(src)
 add_subdirectory(utils)