Make a shell command run async

Author: emmabastas

test/fpm_test/test_lock.f90

@@ -174,7 +174,7 @@ subroutine acquire_blocks(error)
         call setup()
 
         ! Some other process acquires a lock to work on the package briefly.
-        call run('touch .fpm-package-lock && sleep 1 && rm .fpm-package-lock')
+        call run('touch .fpm-package-lock && sleep 1 && rm .fpm-package-lock &')
 
         ! Our blocking acquire should wait for a bit and then go through
         call fpm_lock_acquire(error)