[libc] Make hermetic test depend on the unit test if it exists.

We want to do this so that build system like ninja don't end up running
the hermetic and unit tests in parallel. Running in parallel can cause
problems for tests which read/write disk files as the hermetic and unit
tests can end up stepping on each other.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D151291

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -740,5 +740,12 @@ function(add_libc_test test_name)
   endif()
   if(LIBC_ENABLE_HERMETIC_TESTS AND NOT LIBC_TEST_UNIT_TEST_ONLY)
     add_libc_hermetic_test(${test_name}.__hermetic__ ${LIBC_TEST_UNPARSED_ARGUMENTS})
+    get_fq_target_name(${test_name} fq_test_name)
+    if(TARGET ${fq_test_name}.__unit__)
+      # Tests like the file tests perform file operations on disk file. If we
+      # don't chain up the unit test and hermetic test, then those tests will
+      # step on each other's files.
+      add_dependencies(${fq_test_name}.__hermetic__ ${fq_test_name}.__unit__)
+    endif()
   endif()
 endfunction(add_libc_test)