fix(tests/integration): update plugin output assertion

[vdice]

I was unable to successfully run the integration tests on my machine (Mac Apple Silicon; same with @adamreese I believe), seeing the following:

failures:

---- integration_tests::test_spin_plugin_install_command stdout ----
STDOUT:
Plugin 'example' was installed successfully!

Description:
	A description of the plugin.

STDERR:
Warning: You're using a pre-release version of Spin (2.0.0-pre0). This plugin might not be compatible (supported: >=0.5). Continuing anyway.

STDOUT:
This is an example Spin plugin!

STDERR:
Warning: You're using a pre-release version of Spin (2.0.0-pre0). This plugin might not be compatible (supported: >=0.5). Continuing anyway.

thread 'integration_tests::test_spin_plugin_install_command' panicked at tests/integration.rs:595:9:
assertion `left == right` failed
  left: "\u{1b}(B\u{1b}[m\u{1b}(B\u{1b}[mThis is an example Spin plugin!"
 right: "This is an example Spin plugin!"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    integration_tests::test_spin_plugin_install_command

This proposes tweaking the assertion to use contains instead.