[libFuzzer] Remove hardcoded number of new features in merge_two_step.test.

[Dor1s]

Summary

The number of features can be different on different platforms.

This should fixed broken builders, e.g.
http://lab.llvm.org:8011/builders/clang-cmake-aarch64-full/builds/7946

test/fuzzer/merge_two_step.test

 RUN: %cpp_compiler %S/FullCoverageSetTest.cpp -o %t-FullCoverageSetTest
 
 RUN: rm -rf %t/T0 %t/T1 %t/T2
 RUN: mkdir -p %t/T0 %t/T1 %t/T2
 RUN: echo F..... > %t/T1/1
 RUN: echo .U.... > %t/T1/2
 RUN: echo ..Z... > %t/T1/3
 
 # T1 has 3 elements, T0 is empty.
 RUN: rm -f %t/MCF
 RUN: %run %t-FullCoverageSetTest -merge=1 -merge_control_file=%t/MCF %t/T0 %t/T1 2>&1 | FileCheck %s --check-prefix=CHECK1
 CHECK1: MERGE-OUTER: 3 files, 0 in the initial corpus
-CHECK1: MERGE-OUTER: 3 new files with 11 new features added; 11 new coverage edges
+CHECK1: MERGE-OUTER: 3 new files with {{.*}} new features added; 11 new coverage edges
 
 RUN: echo ...Z.. > %t/T2/1
 RUN: echo ....E. > %t/T2/2
 RUN: echo .....R > %t/T2/3
 RUN: echo F..... > %t/T2/a
 
 RUN: rm -rf %t/T0
 RUN: mkdir -p %t/T0
 
 # T1 has 3 elements, T2 has 4 elements, T0 is empty.
 RUN: %run %t-FullCoverageSetTest -merge=1 -merge_control_file=%t/MCF %t/T0 %t/T1 %t/T2 2>&1 | FileCheck %s --check-prefix=CHECK2
 CHECK2: MERGE-OUTER: non-empty control file provided
 CHECK2: MERGE-OUTER: control file ok, 3 files total, first not processed file 3
 CHECK2: MERGE-OUTER: starting merge from scratch, but reusing coverage information from the given control file
 CHECK2: MERGE-OUTER: 7 files, 0 in the initial corpus, 3 processed earlier
 CHECK2: MERGE-INNER: using the control file 
 CHECK2: MERGE-INNER: 4 total files; 0 processed earlier; will process 4 files now
-CHECK2: MERGE-OUTER: 6 new files with 14 new features added; 14 new coverage edges
+CHECK2: MERGE-OUTER: 6 new files with {{.*}} new features added; 14 new coverage edges