vroom tests fail if clang-format and gofmt aren't installed on host

[dbarnett]

Even though the tests stub out the actual system calls to clang-format and gofmt, they're not completely hermetic because they depend on whether 'clang-format' and 'gofmt' are actual executables on the path on the host where the tests are being run.

This causes vroom failures like

FAILED on line 030: Multiple failures:
Unexpected message:
Not available. codefmt doesn't have a default formatter for this buffer.

Expected system call not received.

Got no chance to inject response: 
 EXPECT:    clang-format -style file -assume-filename .*foo.cc .*2>.* (regex mode)
 STDOUT:    void f() {
          int i;
          SomeFunction(parameter,  // comment
                       i);
        }

Messages:

Not available. codefmt doesn't have a default formatter for this buffer.

Failed command on line 030:
:FormatCode

Queued system controls are:
 EXPECT:    clang-format -style file -assume-filename .*foo.cc .*2>.* (regex mode)
 STDOUT:    void f() {
          int i;
          SomeFunction(parameter,  // comment
                       i);
        }

No system calls received. Perhaps your --shell is broken?

Last few commands (most recent last) were:
:let g:repo = fnamemodify($VROOMFILE, ':p:h:h')<CR>
:execute 'source' g:repo . '/bootstrap.vim'<CR>
:call maktaba#syscall#SetUsableShellRegex('\v<shell\.vroomfaker$')<CR>
:filetype plugin on<CR>
:let g:repeat_calls = []<CR>
:function FakeRepeat(...)<CR>  call add(g:repeat_calls, a:000)<CR><CR>
:endfunction<CR>
:call maktaba#test#Override('repeat#set', 'FakeRepeat')<CR>
:silent file foo.cc<CR>
:set filetype=cpp<CR>
ivoid f() {int i; SomeFunction(parameter,// comment<CR>i);}<ESC>
:FormatCode<CR>

There's no way to actually stub out executable() calls in vroom, so we'll either need to provide some override to actually change the plugin behavior for testing or monkey patch the IsAvailable() function on the formatters we're testing.