Space in setTargetDirName() causes Verilator to fail

**Type of issue**: bug report

**Impact**: no functional change

**Other information**

**If the current behavior is a bug, please provide the steps to reproduce the problem:**

Use space in the filename of the target dir. I discovered this problem without the intention of using space in the filename.

```
TesterOptionsManager {
testerOptions = testerOptions.copy(
backendName = "verilator",
//testerSeed = 1,
isVerbose = true
)
setTargetDirName("test_run_dir/space in filename")
```

**What is the current behavior?**

Failure:

```
bash: line 0: cd: too many arguments
[info] - should handle write pagefault *** FAILED *** (1 second, 910 milliseconds)
[info] java.lang.AssertionError: assertion failed:
[info] at scala.Predef$.assert(Predef.scala:223)
[info] at chisel3.core.assert$.apply(Assert.scala:76)
[info] at chisel3.iotesters.setupVerilatorBackend$.apply(VerilatorBackend.scala:270)
```

**What is the expected behavior?**

Success.

**Please tell us about your environment:**

"chisel3" -> "3.1.5",
"chisel-iotesters" -> "1.2.7"