test-execute: Clarify interaction of PassEnvironment= and MANAGER_USER

@evverx brought up that test-execute runs under MANAGER_USER which
forwards all its environment variables to the services. It turns out it
only forwards those that were in the environment at the time of manager
creation, so this test was still working.

It was still possible to attack it by running something like:
  $ sudo VAR1=a VAR2=b VAR3=c ./test-execute

Prevent that attack by unsetting the three variables explicitly before
creating the manager for the test case.

Also add comments explaining the interactions with MANAGER_USER and,
while it has some caveats, this tests are still valid in that context.

Tested by checking that the test running with the variables set from the
external environment will still pass.

src/test/test-execute.c

@@ -169,6 +169,17 @@ static void test_exec_environmentfile(Manager *m) {
 }
 
 static void test_exec_passenvironment(Manager *m) {
+        /* test-execute runs under MANAGER_USER which, by default, forwards all
+         * variables present in the environment, but only those that are
+         * present _at the time it is created_!
+         *
+         * So these PassEnvironment checks are still expected to work, since we
+         * are ensuring the variables are not present at manager creation (they
+         * are unset explicitly in main) and are only set here.
+         *
+         * This is still a good approximation of how a test for MANAGER_SYSTEM
+         * would work.
+         */
         assert_se(setenv("VAR1", "word1 word2", 1) == 0);
         assert_se(setenv("VAR2", "word3", 1) == 0);
         assert_se(setenv("VAR3", "$word 5 6", 1) == 0);
@@ -274,6 +285,16 @@ int main(int argc, char *argv[]) {
         assert_se(setenv("XDG_RUNTIME_DIR", "/tmp/", 1) == 0);
         assert_se(set_unit_path(TEST_DIR "/test-execute/") >= 0);
 
+        /* Unset VAR1, VAR2 and VAR3 which are used in the PassEnvironment test
+         * cases, otherwise (and if they are present in the environment),
+         * `manager_default_environment` will copy them into the default
+         * environment which is passed to each created job, which will make the
+         * tests that expect those not to be present to fail.
+         */
+        assert_se(unsetenv("VAR1") == 0);
+        assert_se(unsetenv("VAR2") == 0);
+        assert_se(unsetenv("VAR3") == 0);
+
         r = manager_new(MANAGER_USER, true, &m);
         if (IN_SET(r, -EPERM, -EACCES, -EADDRINUSE, -EHOSTDOWN, -ENOENT)) {
                 printf("Skipping test: manager_new: %s", strerror(-r));