`meson test --gdb` launches the wrapper with gdb instead of the wrappee test command

Nobody has claimed this yet.

**Describe the bug**

Using test wrappers will make `--gdb` option not to work, unless the wrapper is a binary itself, but is generally not expected to be tested, while we care about the wrappee.

**To Reproduce**

```meson
project('foo')

add_test_setup('default',
is_default: true,
exe_wrapper: find_program('env'),
)
test('true', find_program('true'))
```

**Expected behavior**

`meson test _build --gdb` will launch:
`/usr/bin/env gdb --quiet --nh --args /bin/true`

While it's currently launching: `gdb --quiet --nh --args /usr/bin/env /bin/true`.

It may probably be even added addressed by using another command-line option (like `--gdb-test-only`) if we don't want to break the previous behavior, but personally I find that using wrappers is breaking hardly the ability to use debuggers with tests.

Run the provided Meson project and compare `meson test _build --gdb` with the expected command invocation. Trace the `--gdb` handling for test setups with `exe_wrapper`; done means the wrapper runs before gdb and gdb receives `/bin/true` as the test command.

Written by the indexing model from the issue text.