tests: insertbin: Don't use duplicated variable name
../subprojects/gst-plugins-bad/tests/check/libs/insertbin.c(46): warning C4273: 'mutex': ...
The "mutex" was declared in gstcheck.h already
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../subprojects/gst-plugins-bad/tests/check/libs/insertbin.c(46): warning C4273: 'mutex': ...
The "mutex" was declared in gstcheck.h already