Fix incorrect use of loop variable in parallel test (#11205)
Fix incorrect use of loop variable in parallel test (#11205)

This fixes an occurrence of a loop variable being captured in a
parallel test (`TestInitialUpdate`). Prior to this commit, only the
last test case declared in that test would actually execute. To work
around this problem, we create a copy of the range variable before the
paralllel test, as suggested in the documentation for the `testing`
package:

https://pkg.go.dev/testing#hdr-Subtests_and_Sub_benchmarks

The test immediately after the one fixed here (`TestInvalidFile`)
followed the same pattern but correctly created a copy of the loop
variable, illustrating how easy it is to forget to do this in
practice.

Issue was automatically found using the `loopvarcapture` linter.

Signed-off-by: Renato Costa <12c52837d5d14706874ffa530176c21f4ca047dd@cockroachlabs.com>

Signed-off-by: Renato Costa <12c52837d5d14706874ffa530176c21f4ca047dd@cockroachlabs.com>
prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus