Double-check how escaping strings works for regex-based CHECKs

[stanislaw]

/sandbox/build/build.mull.dir/tools/driver-cxx/mull-cxx -version
+ /Users/Stanislaw/workspace/code/FileCheck.py/tests/integration/tools/FileCheck/FileCheck /sandbox/mull/tests-lit/tests/tutorials/hello-world/step-1-version/sample.cpp --strict-whitespace --match-full-lines
/sandbox/mull/tests-lit/tests/tutorials/hello-world/step-1-version/sample.cpp:5:9: error: CHECK: expected string not found in input
; CHECK:{{^Version: \d+\.\d+.\d+$}}
        ^
<stdin>:3:1: note: scanning from here
Version: 0.6.1
^

[stanislaw]

This too:

+ : 'RUN: at line 2'
+ /sandbox/build/build.mull.dir/tools/driver-cxx/mull-cxx -version
+ /Users/Stanislaw/workspace/inbox-active-code/FileCheck.py/tests/integration/tools/FileCheck/FileCheck /sandbox/mull/tests-lit/tests/tutorials/hello-world/step-1-version/sample.cpp --strict-whitespace --match-full-lines
/sandbox/mull/tests-lit/tests/tutorials/hello-world/step-1-version/sample.cpp:5:9: error: CHECK: expected string not found in input
; CHECK:{{^Version: \d+\.\d+\.\d+$}}
        ^
<stdin>:3:1: note: scanning from here
Version: 0.6.1
^

and

/sandbox/mull/tests-lit/tests/tutorials/hello-world/step-1-version/sample.cpp:8:9: error: CHECK: expected string not found in input
; CHECK:{{^LLVM: \d+\.\d+.\d+$}}
        ^
<stdin>:6:1: note: scanning from here
LLVM: 9.0.0
^