Disable test caching on Windows, hide gotestsum skipped tests outside CI

[jakebailey]

We've discovered that the go test cache makes go test ./internal/testrunner 4-5x slower on Windows:

$ go clean -testcache
$ Measure-Command { go test ./internal/testrunner | Out-Default }
ok      github.com/microsoft/typescript-go/internal/testrunner  13.584s

Days              : 0
Hours             : 0
Minutes           : 1
Seconds           : 5
Milliseconds      : 364
Ticks             : 653649277
TotalDays         : 0.000756538515046296
TotalHours        : 0.0181569243611111
TotalMinutes      : 1.08941546166667
TotalSeconds      : 65.3649277
TotalMilliseconds : 65364.9277

$ Measure-Command { go test -count=1 ./internal/testrunner | Out-Default }
ok      github.com/microsoft/typescript-go/internal/testrunner  11.726s

Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 15
Milliseconds      : 296
Ticks             : 152960956
TotalDays         : 0.000177038143518519
TotalHours        : 0.00424891544444444
TotalMinutes      : 0.254934926666667
TotalSeconds      : 15.2960956
TotalMilliseconds : 15296.0956

Disable it for hereby test on Windows by setting -count=1 for now until we can report this upstream.

Additionally, we have >1000 skipped tests, which spam the gotestsum output. Outside CI, hide this text by default. (I had originally not used skipped tests for the test runner for this reason, which I could change back again instead. Instead this PR will explicitly print a warning about not having the submodule cloned, which is the only place we had skipped tests previously.)

[jakebailey]

Filed golang/go#72992 upstream.