Fix tmt run --follow, add test coverage for it

Wrong indent in `Run.follow()` introduced in c105887 caused the
`--follow` option to trigger a traceback. Fix the indent and add
basic test coverage to prevent similar regressions in the future.

tests/run/follow/data/.fmf/version

@@ -0,0 +1 @@
+1

tests/run/follow/data/main.fmf

@@ -0,0 +1,10 @@
+/plan:
+    discover:
+        how: fmf
+    provision:
+        how: local
+    execute:
+        how: tmt
+
+/test:
+    test: for i in {00..05}; do echo step-$i; sleep 1; done

tests/run/follow/main.fmf

@@ -0,0 +1 @@
+summary: Ensure that following debug log works

tests/run/follow/test.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+. /usr/share/beakerlib/beakerlib.sh || exit 1
+
+rlJournalStart
+    rlPhaseStartSetup
+        rlRun "run=\$(mktemp -d)" 0 "Create run directory"
+        rlRun "pushd data"
+    rlPhaseEnd
+
+    rlPhaseStartTest
+        rlRun "tmt run --id $run &>/dev/null &" 0 "Start a tmt run in background"
+        rlRun "sleep 1" 0 "Ignore logging during the first second"
+        rlRun -s "timeout -s INT 3 tmt run --id $run --follow" 124 "Follow logs for 3 seconds"
+        rlAssertGrep "step-01" $rlRun_LOG
+        rlAssertGrep "step-02" $rlRun_LOG
+        rlAssertNotGrep "step-04" $rlRun_LOG
+        rlAssertNotGrep "step-05" $rlRun_LOG
+        rlRun "wait $!" 0 "Wait until the run is finished"
+    rlPhaseEnd
+
+    rlPhaseStartCleanup
+        rlRun "popd"
+        rlRun "rm -r $run" 0 "Remove run directory"
+    rlPhaseEnd
+rlJournalEnd

tmt/base.py

@@ -3102,12 +3102,12 @@ def follow(self) -> None:
                 if read_lines > FOLLOW_LINES:
                     break
 
-        while True:
-            line = logfile.readline()
-            if line:
-                print(line, end='')
-            else:
-                time.sleep(0.5)
+            while True:
+                line = logfile.readline()
+                if line:
+                    print(line, end='')
+                else:
+                    time.sleep(0.5)
 
     def go(self) -> None:
         """ Go and do test steps for selected plans """