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
../subprojects/gst-plugins-bad/tests/check/libs/insertbin.c(46): warning C4273: 'mutex': ...
The "mutex" was declared in gstcheck.h already