[llvm-cov] Disable threading in a test. NFC.

PR30735 reports an issue where llvm-cov hangs with a worker thread
waiting on a condition, and the main thread waiting to join() the
workers. While this doesn't appear to be a bug in llvm-cov or the
ThreadPool implementation, it would be helpful to disable the use of
threading in the llvm-cov tests where no test coverage is added.

More context: https://bugs.llvm.org/show_bug.cgi?id=30735

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307610 91177308-0d34-0410-b5e6-96231b3b80d8

test/tools/llvm-cov/zeroFunctionFile.c

@@ -13,7 +13,7 @@ int main() {
 // REPORT: 0                 0         -           0                 0         -               0               0         -           0                 0         -
 // REPORT-NO: 0%
 
-// RUN: llvm-cov show %S/Inputs/zeroFunctionFile.covmapping -format html -instr-profile %t.profdata -o %t.dir
+// RUN: llvm-cov show -j 1 %S/Inputs/zeroFunctionFile.covmapping -format html -instr-profile %t.profdata -o %t.dir
 // RUN: FileCheck %s -input-file=%t.dir/index.html -check-prefix=HTML
 // HTML: <td class='column-entry-green'><pre>- (0/0)
 // HTML-NO: 0.00% (0/0)