index_build.js fails with "SyntaxError: redeclaration of let IndexBuildTest" error in parallel test suite

A couple tests in jstests/core/ were modified to import the jstests/noPassthrough/libs/build_index.js, a JS test helper for index builds. However, tests in jstests/core/ are also run concurrently in the parallel test suite and the modified tests have been failing in the parallel test suite with the following error:

SyntaxError: redeclaration of let IndexBuildTest :

@jstests/noPassthrough/libs/index_build.js:1:1

@jstests/core/apply_ops_index_collation.js:20:1

@jstests/core/apply_ops_index_collation.js:15:2

time<@eval:11:17