roachtest: enable jemalloc profiling in all roachtests

This patch ensures that roachtest clusters start with jemalloc
profiling enabled.

(Ideally we'd like to do this for _all_ cockroachdb servers, however
that would need a more invasive change. Punting until a later PR.)

Release note: None

pkg/cmd/roachtest/cluster.go

@@ -2124,12 +2124,20 @@ func (c *cluster) StartE(ctx context.Context, opts ...option) error {
 	}
 	args = append(args, roachprodArgs(opts)...)
 	args = append(args, c.makeNodes(opts...))
+	// Enable Jemalloc profiles for all roachtests. This has only
+	// minimal performance impact and significantly improves the
+	// troubleshootability of memory-related issues.
+	args = append(args, enableJemallocArgs()...)
 	if !argExists(args, "--encrypt") && c.encryptDefault {
 		args = append(args, "--encrypt")
 	}
 	return execCmd(ctx, c.l, args...)
 }
 
+func enableJemallocArgs() []string {
+	return []string{"-e", "MALLOC_CONF=prof:true"}
+}
+
 // Start is like StartE() except it takes a test and, on error, calls t.Fatal().
 func (c *cluster) Start(ctx context.Context, t *test, opts ...option) {
 	FatalIfErr(t, c.StartE(ctx, opts...))