Use before(:each) rather than before(:all) as recommended [here](https://makandracards.com/makandra/11507-using-before-all-in-rspec-will-cause-you-lots-of-trouble-unless-you-know-what-you-are-doing) to fix Arduino-CI#288.

Author: jgfoster

spec/testsomething_unittests_spec.rb

@@ -72,7 +72,7 @@
         context "file #{tfn} (using #{compiler})" do
           around(:example) { |example| fld.in_pristine_fake_libraries_dir(example) }
 
-          before(:all) do
+          before(:each) do
             @cpp_library = backend.install_local_library(cpp_lib_path)
             @exe = @cpp_library.build_for_test_with_configuration(path, [], compiler, config.gcc_config("uno"))
           end