Refactor script compilation / running & use of helper funcs in test-api.cc.

The tests were using different kind of constructs for achieving the same
thing. This makes refactoring the compilation API more difficult than it should
be.

cctest.h already contained helpers for compiling and running scripts, but they
were not used consistently.

For example, all these were used for running scripts:

v8::Script::Compile(v8_str("foo"))->Run();
v8::Script::Compile(v8::String::NewFromUtf8(isolate, "foo))->Run();
CompileRun(v8_str("foo"));
CompileRun(v8::String::NewFromUtf8(some_way_to_get_isolate(), "foo"));
v8::Local<v8::Script> script = any_of_the_above; script->Run();

Most of the tests just want to run a script (which is in const char*) and don't
care about how the v8::String is constructed or passed to the compiler API. Using
the helpers makes the test more readable and reduces boilerplate code which is
unrelated to what the test is testing.

R=dcarney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/190503002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19753 ce2b1a6d-e550-0410-aec6-3dcde31c8c00