Fix flakey ingester tests

[mdisibio]

What this PR does:
Fixes some flakey ingester tests where the generated test data is a little larger or smaller than before and it's not above or below the tested limits anymore.

Example:

=== RUN   TestInstancePartialSuccess
    instance_test.go:641: 
        	Error Trace:	/Users/marty/src/tempo/modules/ingester/instance_test.go:641
        	Error:      	Not equal: 
        	            	expected: true
        	            	actual  : false
        	Test:       	TestInstancePartialSuccess
--- FAIL: TestInstancePartialSuccess (0.01s)

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]