Speed up tests

by (1) making certain examples ignore uncovered files and (2) using
./spec/test_app as the Bashcov.root_directory for certain other
examples

spec/bashcov/xtrace_spec.rb

@@ -46,9 +46,10 @@ def case_result
 
     context "when shell expansion triggers subshell execution" do
       it "causes extra hits to be reported" do
+        allow(Bashcov).to receive(:skip_uncovered).at_least(:once).and_return(true)
+
         result_without_subshell = case_result
 
-        allow(Bashcov).to receive(:skip_uncovered).and_return(true)
         allow(Bashcov::Xtrace).to receive(:ps4).and_return(subshell_ps4)
 
         result_with_subshell = case_result

spec/bashcov_spec.rb

@@ -18,7 +18,7 @@
 
 describe Bashcov do
   it "preserves the exit status" do
-    system("./bin/bashcov ./spec/test_app/scripts/exit_non_zero.sh")
+    system("./bin/bashcov --root ./spec/test_app ./spec/test_app/scripts/exit_non_zero.sh")
     expect($?.exitstatus).not_to eq(0)
   end