Test against node 15

[cspotcode]

Add 3x new test matrix entries for node 15, against typescript 2.7, latest, and next

Fixed one test case, where we used a regexp to match an error stack from node. Internal node changes meant the stack is slightly different, so I updated the regexp to match.

node 15 comes with npm v7, but npm v7 has some buggy(?) changes to peerDependencies handling. I don't want to deal with this, so I avoid it by downgrading to npm v6.

[codecov]

Codecov Report

Merging #1138 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1138   +/-   ##
=======================================
  Coverage   79.32%   79.32%           
=======================================
  Files           7        7           
  Lines         711      711           
  Branches      158      158           
=======================================
  Hits          564      564           
  Misses         90       90           
  Partials       57       57           

Flag	Coverage Δ
node_10	76.28% <ø> (ø)
node_12_15	76.63% <ø> (ø)
node_12_16	76.63% <ø> (ø)
node_13	79.04% <ø> (ø)
node_14	79.04% <ø> (ø)
node_15	79.04% <ø> (?)
typescript_2_7	78.90% <ø> (ø)
typescript_latest	78.19% <ø> (ø)
typescript_next	78.05% <ø> (ø)
ubuntu	79.04% <ø> (ø)
windows	79.18% <ø> (ø)

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[coveralls]

Coverage increased (+0.9%) to 86.95% when pulling 1869ba1 on ab/test-node-15 into 5a90aac on master.