user-mgr: convert boost::process::child to fork/execv

Boost process v1 is deprecated and removed in the next version
of Boost.  process v2 is async-only and cannot easily be replaced
with the current usages.  Switch to use raw fork/execv calls instead,
which is what boost would have done internally.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ia802cc5428b37dbe428bf1f8befcac168ff3262b
diff --git a/test/execute_cmd_test.cpp b/test/execute_cmd_test.cpp
index 8e38ce8..685899d 100644
--- a/test/execute_cmd_test.cpp
+++ b/test/execute_cmd_test.cpp
@@ -27,8 +27,9 @@
 
 TEST(ExecuteCmdTest, NonExistentCommand)
 {
-    EXPECT_THROW(phosphor::user::executeCmd("/path/to/nonexistent_command"),
-                 boost::process::process_error);
+    EXPECT_THROW(
+        phosphor::user::executeCmd("/path/to/nonexistent_command"),
+        sdbusplus::xyz::openbmc_project::Common::Error::InternalFailure);
 }
 
 TEST(ExecuteCmdTest, CommandReturnsNonZeroExitCode)