all: fix six tests that failed with -count=2

Avery found a bunch of tests that fail with -count=2.

Updates tailscale/corp#40176 (tracks making our CI detect them)

Change-Id: Ie3e4398070dd92e4fe0146badddf1254749cca20
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Co-authored-by: Avery Pennarun <apenwarr@tailscale.com>

ipn/ipnlocal/cert_test.go

@@ -519,8 +519,11 @@ func TestGetCertPEMWithValidity(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 
+			tstest.AssertNotParallel(t)
 			if tt.readOnlyMode {
 				envknob.Setenv("TS_CERT_SHARE_MODE", "ro")
+			} else {
+				envknob.Setenv("TS_CERT_SHARE_MODE", "")
 			}
 
 			os.RemoveAll(certDir)