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You should be able to add conditions to the rule to exclude paths, for example:
<conditions>
<add input="{REQUEST_URI}" pattern="^/umbraco/" negate="true" />
</conditions>
In your case i would look something like (I think you can simplify the match url though):
<rewrite>
<rules>
<rule name="Startview Templating">
<match url="^(http(s)?:\/\/)?(www\.)?([A-Za-z0-9:._-]*)(\/(?!umbraco)([A-Za-z0-9:_-]*))?" />
<conditions>
<add input="{REQUEST_URI}" pattern="^/umbraco/" negate="true" />
</conditions>
<action type="Rewrite" url="{R:0}?alttemplate=test" appendQueryString="true"/>
</rule>
</rules>
</rewrite>
Source:stackexchange.com