Update GC tests to use execute[Limited]TestJava

A lot of our tests use a multi-step recipe to spawn and wait for a process. Here's an example
```
    ProcessBuilder pb = ProcessTools.createTestJavaProcessBuilder(
         "-XX:-UseTLAB",
         "-XX:+UnlockDiagnosticVMOptions",
         "-XX:+VerifyDuringStartup",
         "-Xlog:gc+verify=debug",
         "-version");
    OutputAnalyzer output = new OutputAnalyzer(pb.start());
    ... do something with output and wait for the process to complete ...
```

These are the steps involved:

1) Create a `ProcessBuilder`
2) Call `ProcessBuilder::start`
3) Create an `OutputAnalyzer`
4) Perform an operation that finally waits for the process to, at least partially, complete (OutputAnalyzer::getOutput, OutputAnalyzer::shouldHaveExitValue(), and more).

Almost all our tests could be converted to use a single call to `ProcessTools.executeTestJava` (or `executeLimitedTestJava`), which spawns the process, makes sure that it has fully completed, and then returns a filled-in OutputAnalyzer to the caller. The above example would become:

```
    OutputAnalyzer output = ProcessTools.executeTestJava(
         "-XX:-UseTLAB",
         "-XX:+UnlockDiagnosticVMOptions",
         "-XX:+VerifyDuringStartup",
         "-Xlog:gc+verify=debug",
         "-version");
```

I propose that we make this change in the GC tests, to make our code simpler and hopefully easier to read.