tests: replace remaining usage of tmp.*os.getuid with tmp.*os.getpid(), to make tests more robust against running concurrently from different shells

Author: spytheman

vlib/v/slow_tests/run_project_folders/compile_obj_file_and_link_separately/main.v

@@ -2,7 +2,7 @@ module main
 
 import os
 
-const tfolder = os.join_path(os.vtmp_dir(), 'vobjfile_${os.getuid()}')
+const tfolder = os.join_path(os.vtmp_dir(), 'vobjfile_${os.getpid()}')
 const vexe = os.quoted_path(@VEXE)
 const gcc = os.quoted_path(os.find_abs_path_of_executable('gcc') or {
 	println('This program needs `gcc` to be present.')