gopls/internal/test/marker: remove runtime.GOROOT from format.txt case

The runtime.GOROOT function is about to be marked as deprecated, which
will generate a diagnostic that isn't expected in format.txt test case.
Pick another hopefully somewhat future-proof identifier, since dealing
with this doesn't seem to be in scope of this test case.

For golang/go#51473.

Change-Id: I54d7ed0b43860bcd41938328211797a0cdd60e36
Reviewed-on: https://go-review.googlesource.com/c/tools/+/564275
Auto-Submit: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: dmitshur

gopls/internal/test/marker/testdata/format/format.txt

@@ -43,7 +43,7 @@ func hello() {
 }
 
 func hi() {
-	runtime.GOROOT()
+	runtime.NumCPU()
 	fmt.Printf("")
 
 	log.Printf("")
@@ -63,7 +63,7 @@ func hello() {
 }
 
 func hi() {
-	runtime.GOROOT()
+	runtime.NumCPU()
 	fmt.Printf("")
 
 	log.Printf("")