fix(test): Distinguish 'testname' from escaped arguments

When working on clap v4, it appeared that `last` and `trailing_var_arg`
are mutually exclusive, so I called that out in the debug asserts in
clap-rs/clap#4187.  Unfortunately, I didn't document my research on this
as my focus was elsewhere.

When updating cargo to use clap v4 in #11159, I found `last` and
`trailing_var_arg` were used together.  I figured this was what I was
trying to catch with above and I went off of my intuitive sense of these
commands and assumed `trailing_var_arg` was intended, not knowing about
the `testname` positional that is mostly just a forward to `libtest`,
there for documentation purposes, except for a small optimization.

So it looks like we just need the `last` call and not the
`trailing_var_arg` call.

This restores us to behavior from 531ce1321 which is what I originally
wrote the tests against.

src/bin/cargo/commands/bench.rs

@@ -14,7 +14,7 @@ pub fn cli() -> Command {
             Arg::new("args")
                 .help("Arguments for the bench binary")
                 .num_args(0..)
-                .trailing_var_arg(true),
+                .last(true),
         )
         .arg_targets_all(
             "Benchmark only this package's library",

src/bin/cargo/commands/test.rs

@@ -15,7 +15,7 @@ pub fn cli() -> Command {
             Arg::new("args")
                 .help("Arguments for the test binary")
                 .num_args(0..)
-                .trailing_var_arg(true),
+                .last(true),
         )
         .arg(
             flag(

tests/testsuite/test.rs

@@ -760,6 +760,7 @@ fn pass_through_escaped() {
 [COMPILING] foo v0.0.1 ([CWD])
 [FINISHED] test [unoptimized + debuginfo] target(s) in [..]
 [RUNNING] [..] (target/debug/deps/foo-[..][EXE])
+[DOCTEST] foo
 ",
         )
         .with_stdout_contains("running 1 test")
@@ -771,6 +772,7 @@ fn pass_through_escaped() {
             "\
 [FINISHED] test [unoptimized + debuginfo] target(s) in [..]
 [RUNNING] [..] (target/debug/deps/foo-[..][EXE])
+[DOCTEST] foo
 ",
         )
         .with_stdout_contains("running 1 test")
@@ -782,6 +784,7 @@ fn pass_through_escaped() {
             "\
 [FINISHED] test [unoptimized + debuginfo] target(s) in [..]
 [RUNNING] [..] (target/debug/deps/foo-[..][EXE])
+[DOCTEST] foo
 ",
         )
         .with_stdout_contains("running 2 tests")