[test] Show how unmarshaler detection is possibly broken for embedded structs

[mx-psi]

--- FAIL: TestUnmarshaler (0.00s)
    /home/pablo.baeyens/Source/otel/opentelemetry-collector/confmap/confmap_test.go:373: 
        	Error Trace:	/home/pablo.baeyens/Source/otel/opentelemetry-collector/confmap/confmap_test.go:373
        	Error:      	Not equal: 
        	            	expected: "make sure this is also called"
        	            	actual  : "make sure this"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-make sure this is also called
        	            	+make sure this
        	Test:       	TestUnmarshaler
FAIL
FAIL	go.opentelemetry.io/collector/confmap	0.008s
FAIL

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[github-actions]

Closed as inactive. Feel free to reopen if this PR is still being worked on.